Pioneer Series Trust Xii N-CSRCertified annual shareholder report (management investment company)

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-CSR

CERTIFIED SHAREHOLDER REPORT OF REGISTERED

MANAGEMENT INVESTMENT COMPANIES

Investment Company Act file number 811-08547

Pioneer Series Trust XII

(Exact name of registrant as specified in charter)

60 State Street, Boston, MA 02109

(Address of principal executive offices) (ZIP code)

Terrence J. Cullen, Amundi Asset Management, Inc.,

60 State Street, Boston, MA 02109

(Name and address of agent for service)

Registrant’s telephone number, including area code:  (617) 742-7825

Date of fiscal year end:  August 31, 2021

Date of reporting period: September 1, 2020 through August 31, 2021

Form N-CSR is to be used by management investment companies to file reports with the Commission not later than 10 days after the transmission to stockholders of any report that is required to be transmitted to stockholders under Rule 30e-1 under the Investment Company Act of 1940 (17 CFR 270.30e-1).  The Commission may use the information provided on Form N-CSR in its regulatory, disclosure review, inspection, and policymaking roles.

A registrant is required to disclose the information specified by Form N-CSR, and the Commission will make this information public. A registrant is not required to respond to the collection of information contained in Form N-CSR unless the Form displays a currently valid Office of Management and Budget (“OMB”) control number. Please direct comments concerning the accuracy of the information collection burden estimate and any suggestions for reducing the burden to Secretary, Securities and Exchange Commission, 450 Fifth Street, NW, Washington, DC 20549-0609.  The OMB has reviewed this collection of information under the clearance requirements of 44 U.S.C. ss. 3507.

Pioneer MAP - High Income Municipal Fund

Annual Report | August 31, 2021

Ticker Symbol: HIMUX

Paper copies of the Fund’s shareholder reports are no longer sent by mail, unless you specifically request paper copies of the reports from the Fund or from your financial intermediary, such as a broker-dealer, bank or insurance company. Instead, the reports are available on the Fund’s website, and you will be notified by mail each time a report is posted and provided with a website link to access the report.

You may elect to receive all future reports in paper free of charge. If you invest directly with the Fund, you can inform the Fund that you wish to continue receiving paper copies of your shareholder reports by calling 1-800-225-6292. If you invest through a financial intermediary, you can contact your financial intermediary to request that you continue to receive paper copies of your shareholder reports. Your election to receive reports in paper will apply to all funds held in your account if you invest through your financial intermediary or all funds held within the Pioneer Fund complex if you invest directly.

visit us: www.amundi.com/us

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Table of Contents

President’s Letter

Dear Shareholders,

The past year and a half has created unprecedented challenges for investors, as the COVID-19 pandemic has not only dominated the headlines since March 2020, but has also led to significant changes in government and central-bank policies, both in the US and abroad, and affected the everyday lives of each of us. As we move into the final months of 2021, the situation, while improved, has continued to evolve.

Widespread distribution of the COVID-19 vaccines approved for emergency use in late 2020 led to a general decline in virus-related hospitalizations in the US and had a positive effect on overall market sentiment during the first half of this calendar year. The passage of two additional fiscal stimulus packages by US lawmakers last December and January also helped drive a strong market rally. However, the emergence of highly infectious variants of the virus has caused a recent spike in cases and hospitalizations, especially outside of the US. That development has contributed to a slowdown in the global economic recovery, as some foreign governments have reinstated strict virus-containment measures that had been relaxed after the rollout of the vaccines.

In the US, while performance of most asset classes, especially equities, has been positive for the year to date, volatility has been high, and the third quarter of 2021 saw negative returns for several stock market indices. Investors’ concerns over global supply chain issues, rising inflation, “hawkish” signals concerning less-accommodative future monetary policies from the Federal Reserve System (Fed), partisan debates over spending packages in Washington (particularly infrastructure spending), and the possibility of major tax-policy changes, are among the many factors that have led to greater uncertainty and an increase in market volatility.

Despite those concerns and some of the recent difficulties that have affected the economy and the markets, we believe the distribution of the COVID-19 vaccines has provided a potential light at the end of the pandemic tunnel. With that said, the long-term impact on the global economy from COVID-19, while currently unknown, is likely to be considerable, as it is clear that several industries have already felt greater effects than others, and could continue to struggle for quite some time.

After leaving our offices in March of 2020 due to COVID-19, we have re-opened our US locations and our employees have returned to the office as of mid-October. I am proud of the careful planning that has taken place.

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Throughout the pandemic, our business has continued to operate without any disruption, and we all look forward to regaining a bit of normalcy after so many months of remote working.

Since 1928, Amundi US’s investment process has been built on a foundation of fundamental research and active management, principles which have guided our investment decisions for more than 90 years. We believe active management – that is, making active investment decisions – can help mitigate the risks during periods of market volatility.

At Amundi US, active management begins with our own fundamental, bottom-up research process. Our team of dedicated research analysts and portfolio managers analyzes each security under consideration, communicating directly with the management teams of the companies issuing the securities and working together to identify those securities that best meet our investment criteria for our family of funds. Our risk management approach begins with each and every security, as we strive to carefully understand the potential opportunity, while considering any and all risk factors.

Today, as investors, we have many options. It is our view that active management can serve shareholders well, not only when markets are thriving, but also during periods of market stress.

As you consider your long-term investment goals, we encourage you to work with your financial professional to develop an investment plan that paves the way for you to pursue both your short-term and long-term goals.

We greatly appreciate the trust you have placed in us and look forward to continuing to serve you in the future.

Sincerely,

Lisa M. Jones

Head of the Americas, President and CEO of US

Amundi Asset Management US, Inc.

October 2021

Any information in this shareowner report regarding market or economic trends or the factors influencing the Fund’s historical or future performance are statements of opinion as of the date of this report. Past performance is no guarantee of future results.

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Portfolio Management Discussion | 8/31/21

In the following interview, Jonathan Chirunga and David Eurkus discuss the factors that influenced the performance of Pioneer MAP – High Income Municipal Fund* during the annual reporting period from December 21, 2020 (commencement of operations), through August 31, 2021. Mr. Chirunga, Managing Director, Director of High-Yield Municipals, and a portfolio manager at Amundi Asset Management US, Inc. (Amundi US), is responsible for the day-to-day management of the Fund, along with Mr. Eurkus, Managing Director, Director of Municipals, and a portfolio manager at Amundi US.

Together, those factors helped municipals deliver solid performance over the reporting period, even though US Treasury yields rose. (Bond prices and yields typically move in opposite directions.)

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An ongoing improvement in credit-market conditions provided further support for tax-exempt debt during the period. Like most areas of the financial markets, municipal bonds came under significant stress in early 2020 with the onset of the COVID-19 pandemic and the lockdowns designed to mitigate the spread of the virus. To combat the economic impact of COVID-19, the US Federal Reserve (Fed) reduced the target range of the benchmark federal funds rate to near zero and enacted a quantitative easing (QE), or bond-purchase program. Although investors began to anticipate the “tapering” of QE as the summer of 2021 progressed, the Fed’s aggressive policies remained in place throughout the reporting period. In addition, US lawmakers approved two large fiscal aid packages in the spring and summer of 2020, followed by another $1.9 trillion in stimulus in early 2021. Those measures helped stabilize the economy and – most important for investors in municipal bonds –provided relief to state and local governments.

On the positive side, the Fund’s relative returns benefited from an overweight allocation to the tobacco sector. Tobacco Master Settlement Agreement bonds (tobacco bonds) have been the top-performing area of the municipal market over the last five years, and once again led the market over the annual reporting period. Tobacco bonds also represented the Fund’s largest sector weight as of period-end. We have found tobacco bonds to be attractive investments, not only for their potential to enhance the Fund’s performance, but also for the benefits received by the settling states that issue tobacco bonds since the establishment of the Master Settlement Agreement between the settling states and the tobacco-related companies. Those benefits have included: substantial funding for the advancement of public health; the implementation of important tobacco-related public health measures; and funding towards establishment of a national foundation dedicated to significantly reducing the use of tobacco products among youths.

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The portfolio’s allocation to debt issued by charter schools, which outpaced the broader Bloomberg Index for the period, was another positive contributor to the Fund’s relative performance. The Fund’s returns also benefited from positions in the health care sector, where we have focused on investing in issuers within the affordable care sector rather than in continuing care retirement communities, since the former covers a much broader segment of the population and thus offers a larger market opportunity, in our opinion.

With respect to individual positions, Buckeye Tobacco Settlement Bonds (Ohio), and Metropolitan Pier & Exposition Authority revenue bonds (Illinois) were among the top performers for the Fund during the period, while revenue bonds issued by the Wisconsin Public Finance Authority and the Illinois Finance Authority were notable detractors from relative performance. We have retained the Fund’s holdings of both securities, based on our belief that their fundamentals indicate the recent weakness could be short-term in nature.

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Based on our proprietary research and due diligence, we also have invested the portfolio in select, non-rated municipals that we felt might potentially provide yields in excess of those generated by issues covered by the major rating agencies. In fact, we rely on our own internal evaluations with respect to all of the securities held in the portfolio, rather than depending solely on the evaluations of the rating agencies.

In addition, we believe the major federal infrastructure proposal currently under debate in Washington, if passed into law, may lead to an increased number of attractive tax-exempt investment opportunities, driven by the variety of new construction projects that such legislation could create. Lastly, given the enormous and ongoing need for federal assistance to cope with the lingering economic effects of COVID-19, the US government may finally have to address its rising debt levels, in part by raising taxes on high-income individuals. In fact, the Biden administration has already floated the idea of increases to both capital gains and individual income tax rates. If higher tax rates eventually become reality, that development may further increase demand for municipal bonds.

Consistent with our investment discipline, we anticipate maintaining a focus on intensive, fundamental research into individual bond issues that we choose for inclusion in the portfolio, while maintaining a close watch on any economic factors that could influence the market.

Please refer to the Schedule of Investments on pages 31–47 for a full listing of Fund securities.

All investments are subject to risk, including the possible loss of principal. In the past several years, financial markets have experienced increased volatility and heightened uncertainty. The market prices of securities may go up or

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down, sometimes rapidly or unpredictably, due to general market conditions, such as real or perceived adverse economic, political, or regulatory conditions, recessions, inflation, changes in interest or currency rates, lack of liquidity in the bond markets, the spread of infectious illness or other public health issues or adverse investor sentiment. These conditions may continue, recur, worsen or spread.

Investments in high-yield or lower-rated securities are subject to greater-than-average price volatility, illiquidity and possibility of default.

When interest rates rise, the prices of fixed-income securities held by the Fund will generally fall. Conversely, when interest rates fall, the prices of fixed-income securities held by the Fund will generally rise.

Investments in the Fund are subject to possible loss due to the financial failure of issuers of underlying securities and their inability to meet their debt obligations.

The Fund’s investments, payment obligations and financing terms may be based on floating rates, such as LIBOR (London Interbank Offered Rate). Plans are underway to phase out the use of LIBOR. There remains uncertainty regarding the nature of any replacement rate and the impact of the transition from LIBOR on the Fund, issuers of instruments in which the Fund invests, and financial markets generally.

Prepayment risk is the chance that an issuer may exercise its right to prepay its security, if falling interest rates prompt the issuer to do so. Forced to reinvest the unanticipated proceeds at lower interest rates, the Fund would experience a decline in income and lose the opportunity for additional price appreciation.

The value of municipal securities can be adversely affected by changes in financial condition of municipal issuers, lower revenues, and regulatory and political developments.

The Fund may use derivatives, such as options, futures, inverse floating rate obligations, swaps, and others, which can be illiquid, may disproportionately increase losses, and have a potentially large impact on Fund performance. Derivatives may have a leveraging effect on the Fund.

A portion of income may be subject to local, state, federal, and/or alternative minimum tax. Capital gains, if any, are subject to a capital gains tax.

At times, the Fund’s investments may represent industries or industry sectors that are interrelated or have common risks, making the Fund more susceptible to any economic, political, or regulatory developments or other risks affecting those industries and sectors.

These risks may increase share price volatility.

Before investing, consider the product’s investment objectives, risks, charges and expenses. Contact your financial professional or Amundi Asset Management US, Inc., for a prospectus or summary prospectus containing this information. Read it carefully.

Any information in this shareholder report regarding market or economic trends or the factors influencing the Fund’s historical or future performance are statements of opinion as of the date of this report. Past performance is not a guarantee of future results.

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Portfolio Summary | 8/31/21

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Prices and Distributions | 8/31/21

* The Fund commenced operations on December 21, 2020.

Index Definitions

The Bloomberg U.S. Municipal High Yield Bond Index is an unmanaged measure of the performance of the high-yield municipal bond market. Index returns are calculated monthly, assume reinvestment of dividends and, unlike Fund returns, do not reflect any fees, expenses or sales charges. It is not possible to invest directly in an index.

The index defined here pertains to the “Value of $10,000 Investment” and “Value of $10,000 Investment” charts shown on page 12.

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Performance Update | 8/31/21

Investment Returns

The mountain chart on the right shows the change in value of a $10,000 investment made in Pioneer MAP - High Income Municipal Fund during the period shown, compared to that of the Bloomberg U.S. Municipal High Yield Bond Index.

Call 1-800-225-6292 or visit www.amundi.com/us for the most recent month-end performance results. Current performance may be lower or higher than the performance data quoted.

The performance data quoted represents past performance, which is no guarantee of future results. Investment return and principal value will fluctuate, and shares, when redeemed, may be worth more or less than their original cost.

All results are historical and assume the reinvestment of dividends and capital gains.

Performance results reflect any applicable expense waivers in effect during the periods shown. Without such waivers Fund performance would be lower. Waivers may not be in effect for all funds. Certain fee waivers are contractual through a specified period. Otherwise, fee waivers can be rescinded at any time. See the prospectus and financial statements for more information.

The net expense ratio reflects the contractual expense limitation currently in effect through January 1, 2022 for Pioneer MAP - High Income Municipal Fund. There can be no assurance that Amundi US will extend the expense limitation beyond such time. Please see the prospectus and financial statements for more information.

The performance table and graph do not reflect the deduction of fees and taxes that a shareowner would pay on Fund distributions or the redemption of Fund shares.

Please refer to the financial highlights for more current expense ratios.

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Comparing Ongoing Fund Expenses

As a shareowner in the Fund, you incur two types of costs:

This example is intended to help you understand your ongoing expenses (in dollars) of investing in the Fund and to compare these costs with the ongoing costs of investing in other mutual funds. The example is based on an investment of $1,000 at the beginning of the Fund’s latest six-month period and held throughout the six months.

Using the Tables

Actual Expenses

The first table below provides information about actual account values and actual expenses. You may use the information in this table, together with the amount you invested, to estimate the expenses that you paid over the period as follows:

Example: an $8,600 account value ÷ $1,000 = 8.6

Expenses Paid on a $1,000 Investment in Pioneer MAP - High Income Municipal Fund*

Based on actual returns from March 1, 2021 through August 31, 2021.

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Hypothetical Example for Comparison Purposes

The table below provides information about hypothetical account values and hypothetical expenses based on the Fund’s actual expense ratio and an assumed rate of return of 5% per year before expenses, which is not the Fund’s actual return. The hypothetical account values and expenses may not be used to estimate the actual ending account balance or expenses you paid for the period.

You may use this information to compare the ongoing costs of investing in the Fund and other funds. To do so, compare this 5% hypothetical example with the 5% hypothetical examples that appear in the shareholder reports of the other funds.

Please note that the expenses shown in the tables are meant to highlight your ongoing costs only and do not reflect any transaction costs, such as sales charges (loads) that are charged at the time of the transaction. Therefore, the table below is useful in comparing ongoing costs only and will not help you determine the relative total costs of owning different funds. In addition, if these transaction costs were included, your costs would have been higher.

Expenses Paid on a $1,000 Investment in Pioneer MAP - High Income Municipal Fund*

Based on a hypothetical 5% return per year before expenses, reflecting the period from March 1, 2021 through August 31, 2021.

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Statement of Assets and Liabilities | 8/31/21

The accompanying notes are an integral part of these financial statements. Additionally, the financial statements of the Pioneer High Income Municipal Portfolio are contained elsewhere in this report and should be read in conjunction with the Fund’s financial statements.

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Statement of Operations

FOR THE PERIOD FROM 12/21/20 TO 8/31/21*

The accompanying notes are an integral part of these financial statements. Additionally, the financial statements of the Pioneer High Income Municipal Portfolio are contained elsewhere in this report and should be read in conjunction with the Fund’s financial statements.

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Statement of Changes in Net Assets

The accompanying notes are an integral part of these financial statements. Additionally, the financial statements of the Pioneer High Income Municipal Portfolio are contained elsewhere in this report and should be read in conjunction with the Fund’s financial statements.

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Financial Highlights

The accompanying notes are an integral part of these financial statements. Additionally, the financial statements of the Pioneer High Income Municipal Portfolio are contained elsewhere in this report and should be read in conjunction with the Fund’s financial statements.

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Notes to Financial Statements | 8/31/21

1. Organization and Significant Accounting Policies

Pioneer MAP-High Income Municipal Fund (the “Fund”) is one of two portfolios comprising Pioneer Series Trust XII (the “Trust”), a Delaware statutory trust. The Fund is registered under the Investment Company Act of 1940 as a diversified, open-end management investment company. The investment objective of the Fund is to maximize total return through a combination of income that is exempt from regular federal income tax and capital appreciation. The Fund commenced operations on December 21, 2020.

The Fund invests all of its investable assets as a feeder fund in Pioneer High Income Municipal Portfolio (the “Portfolio”), a portfolio of Pioneer Core Trust I, that has the same investment objective and policies as the Fund. The financial statements of the Portfolio, including the Schedule of Investments, are contained elsewhere in this report and should be read in conjunction with the Fund’s financial statements. The Portfolio is registered under the Investment Company Act of 1940 as a diversified, open-end management investment company. At August 31, 2021, the Fund owned approximately 0.001% of the Portfolio. The performance of the Fund is directly affected by the performance of the Portfolio. The financial statements of the Portfolio, which are attached, are an integral part of these financial statements. Refer to the accounting policies disclosed in the financial statements of Portfolio for additional information regarding significant accounting policies that affect the Fund.

The Fund offers a single class of shares. The Fund’s shares are offered through Amundi Distributor US, Inc. (the “Distributor”), an affiliate of the Adviser. Shares are offered at the Fund’s current net asset value (“NAV”) per share. The Amended and Restated Declaration of Trust of the Trust gives the Board of Trustees the flexibility to specify either per-share voting or dollar-weighted voting when submitting matters for shareowner approval. Under per-share voting, each share of the Fund is entitled to one vote. Under dollar-weighted voting, a shareowner’s voting power is determined not by the number of shares owned, but by the dollar value of the shares on the record date.

Amundi Asset Management US, Inc., an indirect, wholly owned subsidiary of Amundi and Amundi’s wholly owned subsidiary, Amundi USA, Inc., serves as the Fund’s investment adviser (the “Adviser”). Prior to January 1, 2021, the Adviser was named Amundi Pioneer Asset Management, Inc.

In August 2018, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update 2018-13 “Disclosure Framework — Changes to the Disclosure Requirements for Fair Value Measurement” (“ASU 2018-13”)

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which modifies disclosure requirements for fair value measurements, principally for Level 3 securities and transfers between levels of the fair value hierarchy. ASU 2018-13 is effective for fiscal years beginning after December 15, 2019 and for interim periods within those fiscal years. The Fund has adopted ASU 2018-13 for the period ended August 31, 2021. The impact to the Fund’s adoption was limited to changes in the Fund’s disclosures regarding fair value, primarily those disclosures related to transfers between levels of the fair value hierarchy and disclosure of the range and weighted average used to develop significant unobservable inputs for Level 3 fair value investments, when applicable.

In March 2020, FASB issued an Accounting Standard Update, ASU 2020-04, Reference Rate Reform (Topic 848) — Facilitation of the Effects of Reference Rate Reform on Financial Reporting (“ASU 2020-04”), which provides optional, temporary relief with respect to the financial reporting of contracts subject to certain types of modifications due to the planned discontinuation of the London Interbank Offered Rate (“LIBOR”) and other LIBOR-based reference rates at the end of 2021. The temporary relief provided by ASU 2020-04 is effective for certain reference rate-related contract modifications that occur during the period from March 12, 2020 through December 31, 2022. Management is evaluating the impact of ASU 2020-04 on the Fund’s investments, derivatives, debt and other contracts, if applicable, that will undergo reference rate-related modifications as a result of the reference rate reform.

The Fund is an investment company and follows investment company accounting and reporting guidance under U.S. Generally Accepted Accounting Principles (“U.S. GAAP”). U.S. GAAP requires the management of the Fund to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of income, expenses and gain or loss on investments during the reporting period. Actual results could differ from those estimates.

The following is a summary of significant accounting policies followed by the Fund in the preparation of its financial statements:

The net asset value of the Fund is computed once daily, on each day the New York Stock Exchange (“NYSE”) is open, as of the close of regular trading on the NYSE.

The Fund records its investment in the Portfolio at value, which reflects its proportionate interest in the net assets of the Portfolio and is categorized as Level 1. Valuation of the securities held by the Portfolio is discussed in the notes to the Portfolio’s financial statements included elsewhere in this report. Disclosure about the classification of fair value measurements is presented in a tabular format following the Portfolio’s Schedule of Investments.

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The Fund declares as daily dividends substantially all of its net investment income. All dividends are paid on a monthly basis. Short-term capital gain distributions, if any, may be declared with the daily dividends. Distributions to shareowners are recorded as of the ex-dividend date.

The Fund receives a daily allocation of the Portfolio’s income, expenses and net realized and unrealized gains and losses in proportion to its investment in the Portfolio.

It is the Fund’s policy to comply with the requirements of the Internal Revenue Code applicable to regulated investment companies and to distribute all of its net taxable income and net realized capital gains, if any, to its shareowners. Therefore, no provision for federal income taxes is required. As of August 31, 2021, the Fund did not accrue any interest or penalties with respect to uncertain tax positions, which, if applicable, would be recorded as an income tax expense on the Statement of Operations. Tax returns filed within the prior three years remain subject to examination by federal and state tax authorities.

The amount and character of income and capital gain distributions to shareowners are determined in accordance with federal income tax rules, which may differ from U.S. GAAP. Distributions in excess of net investment income or net realized gains are temporary over distributions for financial statement purposes resulting from differences in the recognition or classification of income or distributions for financial statement and tax purposes. Capital accounts within the financial statements are adjusted for permanent book/tax differences to reflect tax character, but are not adjusted for temporary differences.

At August 31, 2021, the Fund reclassified $0 to increase/decrease distributable earnings and $0 to increase/decrease paid-in capital to reflect permanent book/tax differences. These adjustments have no impact on net assets or the results of operations.

At August 31, 2021, the Fund was permitted to carry forward indefinitely $0 of long-term losses.

During the period ended August 31, 2021, a capital loss carryforward of $0 was utilized to offset net realized gains by the Fund.

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The tax character of distributions paid during the period ended August 31, 2021 was as follows:

The following shows the components of distributable earnings (losses) on a federal income tax basis at August 31, 2021:

The Fund invests directly in the Portfolio, the following are the underlying risks of the Portfolio's securities. The value of securities held by the Fund may go up or down, sometimes rapidly or unpredictably, due to general market conditions, such as real or perceived adverse economic, political or regulatory conditions, recessions, the spread of infectious illness or other public health issues, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment. In the past several years, financial markets have experienced increased volatility, depressed valuations, decreased liquidity and heightened uncertainty. These conditions may continue, recur, worsen or spread. A general rise in interest rates could adversely affect the price and liquidity of fixed-income securities and could also result in increased redemptions from the Fund.

At times, the Fund’s investments may represent industries or industry sectors that are interrelated or have common risks, making the Fund more susceptible to any economic, political, or regulatory developments or other risks affecting those industries and sectors. The Fund’s investments in foreign markets and countries with limited developing markets may subject

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the Fund to a greater degree of risk than investments in a developed market. These risks include disruptive political or economic conditions and the imposition of adverse governmental laws or currency exchange restrictions.

The Fund invests in below-investment-grade (high-yield) debt securities and preferred stocks. Some of these high-yield securities may be convertible into equity securities of the issuer. Debt securities rated below investment-grade are commonly referred to as “junk bonds” and are considered speculative. These securities involve greater risk of loss, are subject to greater price volatility, and are less liquid, especially during periods of economic uncertainty or change, than higher rated debt securities.

The municipal bond market can be susceptible to unusual volatility, particularly for lower-rated and unrated securities. Liquidity can be reduced unpredictably in response to overall economic conditions or credit tightening. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities, potentially resulting in defaults. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors. Municipal securities may be more susceptible to downgrades or defaults during recessions or similar periods of economic stress.

Financial difficulties of municipal issuers may continue or get worse. To the extent the Fund invests significantly in a single state, including California, Illinois, New York and Indiana, or in securities the payments on which are dependent upon a single project or source of revenues, or that relate to a sector or industry, including health care facilities, education, transportation, special revenues and pollution control, the Fund will be more susceptible to associated risks and developments.

The Fund’s investments, payment obligations and financing terms may be based on floating rates, such as LIBOR (London Interbank Offered Rate). Plans are underway to phase out the use of LIBOR. The UK Financial Conduct Authority (“FCA”) and LIBOR’s administrator, ICE Benchmark Administration (“IBA”), have announced that most LIBOR rates will no longer be published after the end of 2021 and a majority of U.S. dollar LIBOR rates will no longer be published after June 30, 2023. It is possible that the FCA may compel the IBA to publish a subset of LIBOR settings after these dates on a “synthetic” basis, but any such publications would

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be considered non-representative of the underlying markets. Actions by regulators have resulted in the establishment of alternative reference rates to LIBOR in most major currencies. Based on the recommendations of the New York Federal Reserve’s Alternative Reference Rate Committee (comprised of major derivative market participants and their regulators), the U.S. Federal Reserve began publishing a Secured Overnight Funding Rate (“SOFR”) that is intended to replace U.S. Dollar LIBOR. Proposals for alternative reference rates for other currencies have also been announced or have already begun publication, such as SONIA in the United Kingdom. Markets are slowly developing in response to these new rates, and transition planning is at a relatively early stage. Neither the effect of the transition process nor its ultimate success is known. The transition process may lead to increased volatility and illiquidity in markets that currently rely on LIBOR to determine interest rates. The effect of any changes to — or discontinuation of — LIBOR on the portfolio will vary depending on, among other things, provisions in individual contracts and whether, how, and when industry participants develop and adopt new reference rates and alternative reference rates for both legacy and new products and instruments. Because the usefulness of LIBOR as a benchmark may deteriorate during the transition period, these effects could materialize prior to the end of 2021.

With the increased use of technologies such as the Internet to conduct business, the Fund is susceptible to operational, information security and related risks. While the Fund’s Adviser has established business continuity plans in the event of, and risk management systems to prevent, limit or mitigate, such cyber-attacks, there are inherent limitations in such plans and systems, including the possibility that certain risks have not been identified. Furthermore, the Fund cannot control the cybersecurity plans and systems put in place by service providers to the Fund such as Brown Brothers Harriman & Co., the Fund’s custodian and accounting agent, and DST Asset Manager Solutions, Inc., the Fund’s transfer agent. In addition, many beneficial owners of Fund shares hold them through accounts at broker-dealers, retirement platforms and other financial market participants over which neither the Fund nor the Adviser exercises control. Each of these may in turn rely on service providers to them, which are also subject to the risk of cyber-attacks. Cybersecurity failures or breaches at the Adviser or the Fund’s service providers or intermediaries have the ability to cause disruptions and impact business operations, potentially resulting in financial losses, interference with the Fund’s ability to calculate its net asset value, impediments to trading, the inability of Fund shareowners to effect share purchases, redemptions or exchanges, or receive distributions, loss of or unauthorized access to private shareowner information and violations of applicable privacy and other laws, regulatory fines, penalties, reputational

24 Pioneer MAP - High Income Municipal Fund | Annual Report | 8/31/21

damage, or additional compliance costs. Such costs and losses may not be covered under any insurance. In addition, maintaining vigilance against cyber-attacks may involve substantial costs over time, and system enhancements may themselves be subject to cyber-attacks. 

COVID-19

The global pandemic of the novel coronavirus respiratory disease designated COVID-19 has resulted in major disruption to economies and markets around the world, including the United States. Global financial markets have experienced extreme volatility and severe losses, and trading in many instruments has been disrupted. Liquidity for many instruments has been greatly reduced for periods of time. Some interest rates are very low and in some cases yields are negative. Some sectors of the economy and individual issuers have experienced particularly large losses. Rates of inflation have recently risen. These circumstances may continue for an extended period of time, and may continue to affect adversely the value and liquidity of the Fund’s investments. Governments and central banks, including the Federal Reserve in the U.S., have taken extraordinary and unprecedented actions to support local and global economies and the financial markets. These actions have resulted in significant expansion of public debt, including in the U.S. The impact of these measures will not be known for some time. The consequences of high public debt, including its future impact on the economy and securities markets, likewise may not be known for some time.

The Fund’s prospectus contains unaudited information regarding the Fund’s principal risks. Please refer to that document when considering the Fund’s principal risks.

The Adviser has agreed to waive all expenses for the Fund. Fees waived and expenses reimbursed during the period ended August 31, 2021 are reflected on the Statement of Operations.

The Fund pays an annual fee to its Trustees. The Adviser reimburses the Fund for fees paid to the Interested Trustees. The Fund does not pay any salary or other compensation to its officers. For the period from December 21, 2020 (commencement of operations) through August 31, 2021, the Fund did not pay Trustees’ compensation.

DST Asset Manager Solutions, Inc. serves as the transfer agent to the Fund at negotiated rates. Transfer agent fees and payables are shown on the Statement of Operations and the Statement of Assets and Liabilities, respectively.

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Report of Independent Registered Public Accounting Firm

To the Board of Trustees of Pioneer Series Trust XII and the Shareowners of Pioneer MAP-High Income Municipal Fund:

Opinion on the Financial Statements

We have audited the accompanying statement of assets and liabilities of Pioneer MAP-High Income Municipal Fund (the “Fund”) (one of the funds constituting Pioneer Series Trust XII (the “Trust”)), and the related statement of operations, the statement of changes in net assets and the financial highlights for the period from December 21, 2020 (commencement of operations) through August 31, 2021 and the related notes (collectively referred to as the “financial statements”). In our opinion, the financial statements present fairly, in all material respects, the financial position of Pioneer MAP-High Income Municipal Fund (one of the funds constituting Pioneer Series Trust XII) at August 31, 2021, and the results of its operations, the changes in its net assets and its financial highlights for the period from December 21, 2020 (commencement of operations) through August 31, 2021, in conformity with U.S. generally accepted accounting principles.

Basis for Opinion

These financial statements are the responsibility of the Trust’s management. Our responsibility is to express an opinion on the Fund’s financial statements based on our audit. We are a public accounting firm registered with the Public Company Accounting Oversight Board (United States) (“PCAOB”) and are required to be independent with respect to the Company in accordance with the U.S. federal securities law and the applicable rules and regulations of the Securities and Exchange Commission and the PCAOB.

We conducted our audit in accordance with the standards of the PCAOB. Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements are free of material misstatement, whether due to error or fraud. The Trust is not required to have, nor were we engaged to perform, an audit of the Trust’s internal control over financial reporting. As part of our audit, we are required to obtain an understanding of internal control over financial reporting, but not for the purpose of expressing an opinion on the effectiveness of the Trust’s internal control over financial reporting. Accordingly, we express no such opinion.

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Our audit included performing procedures to assess the risks of material misstatement of the financial statements, whether due to error or fraud, and performing procedures that respond to those risks. Such procedures included examining, on a test basis, evidence regarding the amounts and disclosures in the financial statements. Our audit also included evaluating the accounting principles used and significant estimates made by management, as well as evaluating the overall presentation of the financial statements. We believe that our audit provides a reasonable basis for our opinion.

We have served as the auditor of one or more investment companies in the Pioneer family of funds since 2017.

Boston, Massachusetts
November 4, 2021

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Statement Regarding Liquidity Risk Management Program

As required by law, the Fund has adopted and implemented a liquidity risk management program (the “Program”) that is designed to assess and manage liquidity risk. Liquidity risk is the risk that the Fund could not meet requests to redeem its shares without significant dilution of remaining investors’ interests in the Fund. The Fund’s Board of Trustees designated a liquidity risk management committee (the “Committee”) consisting of employees of Amundi Asset Management US, Inc. (the “Adviser”) to administer the Program.

The Committee provided the Board of Trustees with a report that addressed the operation of the Program and assessed its adequacy and effectiveness of implementation (the “Report”). The Report covered the period from December 21, 2020 (commencement of operations) through December 31, 2020 (the “Reporting Period”).

The Report confirmed that, throughout the Reporting Period, the Committee had monitored the Fund’s portfolio liquidity and liquidity risk on an ongoing basis, as described in the Program and in Board reporting throughout the Reporting Period.

The Report discussed the Committee’s annual review of the Program, which addressed, among other things, the following elements of the Program:

The Committee reviewed the Fund’s investment strategy and liquidity of portfolio investments during both normal and reasonably foreseeable stressed conditions. The Committee noted that the Fund’s investment strategy continues to be appropriate for an open-end fund, taking into account, among other things, whether and to what extent the Fund held less liquid and illiquid assets and the extent to which any such investments affected the Fund’s ability to meet redemption requests. In managing and reviewing the Fund’s liquidity risk, the Committee also considered the extent to which the Fund’s investment strategy involves a relatively concentrated portfolio or large positions in particular issuers, the extent to which the Fund uses borrowing for investment purposes, and the extent to which the Fund uses derivatives (including for hedging purposes). The Committee also reviewed the Fund’s short-term and long-term cash flow projections during both normal and reasonably foreseeable stressed conditions. In assessing the Fund’s cash flow projections, the Committee considered, among other factors, historical net redemption activity, redemption policies, ownership concentration, distribution channels, and the degree of certainty associated with the Fund’s short-term and long-term cash flow projections. The Committee also considered the Fund’s holdings of cash and cash equivalents,

28 Pioneer MAP - High Income Municipal Fund | Annual Report | 8/31/21

as well as borrowing arrangements and other funding sources, including, if applicable, the Fund’s participation in a credit facility, as components of the Fund’s ability to meet redemption requests. The Fund has adopted an in-kind redemption policy which may be utilized to meet larger redemption requests.

The Committee reviewed the Program’s liquidity classification methodology for categorizing the Fund’s investments into one of four liquidity buckets. In reviewing the Fund’s investments, the Committee considered, among other factors, whether trading varying portions of a position in a particular portfolio investment or asset class in sizes the Fund would reasonably anticipate trading, would be reasonably expected to significantly affect liquidity.

The Committee performed an analysis to determine whether the Fund is required to maintain a Highly Liquid Investment Minimum, and determined that no such minimum is required because the Fund primarily holds highly liquid investments.

The Report stated that the Committee concluded the Program operates adequately and effectively, in all material respects, to assess and manage the Fund’s liquidity risk throughout the Reporting Period.

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(The following financial statements of Pioneer High Income Municipal Portfolio should be read in conjunction with the Fund’s financial statements.)

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Schedule of Investments | 8/31/21

Pioneer High Income Municipal Portfolio

The accompanying notes are an integral part of these financial statements.

Pioneer High Income Municipal Portfolio | Annual Report | 8/31/21 31

Schedule of Investments | 8/31/21 (continued)

The accompanying notes are an integral part of these financial statements.

32 Pioneer High Income Municipal Portfolio | Annual Report | 8/31/21

The accompanying notes are an integral part of these financial statements.

Pioneer High Income Municipal Portfolio | Annual Report | 8/31/21 33

Schedule of Investments | 8/31/21 (continued)

The accompanying notes are an integral part of these financial statements.

34 Pioneer High Income Municipal Portfolio | Annual Report | 8/31/21

The accompanying notes are an integral part of these financial statements.

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Schedule of Investments | 8/31/21 (continued)

The accompanying notes are an integral part of these financial statements.

36 Pioneer High Income Municipal Portfolio | Annual Report | 8/31/21

The accompanying notes are an integral part of these financial statements.

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Schedule of Investments | 8/31/21 (continued)

The accompanying notes are an integral part of these financial statements.

38 Pioneer High Income Municipal Portfolio | Annual Report | 8/31/21

The accompanying notes are an integral part of these financial statements.

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Schedule of Investments | 8/31/21 (continued)

The accompanying notes are an integral part of these financial statements.

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The accompanying notes are an integral part of these financial statements.

Pioneer High Income Municipal Portfolio | Annual Report | 8/31/21 41

Schedule of Investments | 8/31/21 (continued)

The accompanying notes are an integral part of these financial statements.

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The accompanying notes are an integral part of these financial statements.

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Schedule of Investments | 8/31/21 (continued)

The accompanying notes are an integral part of these financial statements.

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The accompanying notes are an integral part of these financial statements.

Pioneer High Income Municipal Portfolio | Annual Report | 8/31/21 45

Schedule of Investments | 8/31/21 (continued)

Purchases and sales of securities (excluding temporary cash investments) for the period ended August 31, 2021, aggregated $387,047,349 and $200,060,650, respectively.

The Portfolio is permitted to engage in purchase and sale transactions (“cross trades”) with certain funds and accounts for which Amundi Asset Management US, Inc. (the “Adviser”) serves as the Portfolio’s investment adviser, as set forth in Rule 17a-7 under the Investment Company Act of 1940, pursuant to procedures adopted by the Board of Trustees. Under these procedures, cross trades are effected at current market prices. During the period ended August 31, 2021, the Portfolio did not engage in any cross trade activity.

The accompanying notes are an integral part of these financial statements.

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Various inputs are used in determining the value of the Portfolio’s investments. These inputs are summarized in the three broad levels below.

Level 1 – unadjusted quoted prices in active markets for identical securities.

Level 2 – other significant observable inputs (including quoted prices for similar securities, interest rates, prepayment speeds, credit risks, etc.).

Level 3 – significant unobservable inputs (including the Portfolio’s own assumptions in determining fair value of investments).

The following is a summary of the inputs used as of August 31, 2021, in valuing the Portfolio’s investments:

During the period ended August 31, 2021, there were no transfers in or out of Level 3.

The accompanying notes are an integral part of these financial statements.

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Statement of Assets and Liabilities | 8/31/21

The accompanying notes are an integral part of these financial statements.

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Statement of Operations

FOR THE PERIOD FROM 12/21/20 TO 8/31/21*

* The Portfolio commenced operations on December 21, 2020.

The accompanying notes are an integral part of these financial statements.

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Statement of Changes in Net Assets

The accompanying notes are an integral part of these financial statements.

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Financial Highlights

The accompanying notes are an integral part of these financial statements.

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Notes to Financial Statements | 8/31/21

1. Organization and Significant Accounting Policies

Pioneer High Income Municipal Portfolio (the “Portfolio”) is a diversified series of Pioneer Core Trust I (the “Trust”), an open-end management investment company established as a Delaware statutory trust on October 14, 2020. The Portfolio is registered under the Investment Company Act of 1940 as a diversified, open-end management investment company. The investment objective of the Portfolio is to maximize total return through a combination of income that is exempt from regular federal income tax and capital appreciation.

The Declaration of Trust permits the Trustees to issue beneficial interests in the Portfolio. At August 31, 2021, all investors in the Portfolio were funds advised by the investment adviser of the Portfolio. At August 31, 2021, the Pioneer High Income Municipal Fund owned approximately 99.999% of the Portfolio, and Pioneer MAP - High Income Municipal Fund owned approximately 0.001% of the Portfolio. On December 21, 2020, Pioneer High Income Municipal Fund transferred all of its investable assets, with a cost basis of $1,707,664,760 and a value of $1,760,998,235, to the Portfolio in exchange for an interest in the Portfolio. The transaction was structured to qualify as a tax-free exchange of assets.

Amundi Asset Management US, Inc., an indirect, wholly owned subsidiary of Amundi and Amundi’s wholly owned subsidiary, Amundi USA, Inc., serves as the Portfolio’s investment adviser (the “Adviser”). Prior to January 1, 2021, the Adviser was named Amundi Pioneer Asset Management, Inc. Amundi Distributor US, Inc., an affiliate of the Adviser, serves as the Portfolio’s placement agent (the “Placement Agent”). The Portfolio offers beneficial interests through the Placement Agent.

In August 2018, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update 2018-13 “Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement” (“ASU 2018-13”) which modifies disclosure requirements for fair value measurements, principally for Level 3 securities and transfers between levels of the fair value hierarchy. ASU 2018-13 is effective for fiscal years beginning after December 15, 2019 and for interim periods within those fiscal years. The Portfolio has adopted ASU 2018-13 for the period ended August 31, 2021. The impact to the Portfolio’s adoption was limited to changes in the Portfolio’s disclosures regarding fair value, primarily those disclosures related to transfers between levels of the fair value hierarchy and disclosure of the range and weighted average used to develop significant unobservable inputs for Level 3 fair value investments, when applicable.

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In March 2020, FASB issued an Accounting Standard Update, ASU 2020-04, Reference Rate Reform (Topic 848) — Facilitation of the Effects of Reference Rate Reform on Financial Reporting (“ASU 2020-04”), which provides optional, temporary relief with respect to the financial reporting of contracts subject to certain types of modifications due to the planned discontinuation of the London Interbank Offered Rate (“LIBOR”) and other LIBOR-based reference rates at the end of 2021. The temporary relief provided by ASU 2020-04 is effective for certain reference rate-related contract modifications that occur during the period from March 12, 2020 through December 31, 2022. Management is evaluating the impact of ASU 2020-04 on the Portfolio’s investments, derivatives, debt and other contracts, if applicable, that will undergo reference rate-related modifications as a result of the reference rate reform.

The Portfolio is an investment company and follows investment company accounting and reporting guidance under U.S. Generally Accepted Accounting Principles (“U.S. GAAP”). U.S. GAAP requires the management of the Portfolio to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements, and the reported amounts of income, expenses and gain or loss on investments during the reporting period. Actual results could differ from those estimates.

The following is a summary of significant accounting policies followed by the Portfolio in the preparation of its financial statements:

Investments are stated at value, computed once daily, on each day the New York Stock Exchange (“NYSE”) is open, as of the close of regular trading on the NYSE.

Fixed-income securities are valued by using prices supplied by independent pricing services, which consider such factors as market prices, market events, quotations from one or more brokers, Treasury spreads, yields, maturities and ratings, or may use a pricing matrix or other fair value methods or techniques to provide an estimated value of the security or instrument. A pricing matrix is a means of valuing a debt security on the basis of current market prices for other debt securities, historical trading patterns in the market for fixed-income securities and/or other factors. Non-U.S. debt securities that are listed on an exchange will be valued at the bid price obtained from an independent third party pricing service.

When independent third party pricing services are unable to supply prices, or when prices or market quotations are considered to be unreliable, the value of that security may be determined using quotations from one or more broker-dealers.

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Securities for which independent pricing services or broker-dealers are unable to supply prices or for which market prices and/or quotations are not readily available or are considered to be unreliable are valued by a fair valuation team comprised of certain personnel of the Adviser pursuant to procedures adopted by the Portfolio’s Board of Trustees. The Adviser’s fair valuation team uses fair value methods approved by the Valuation Committee of the Board of Trustees. The Adviser’s fair valuation team is responsible for monitoring developments that may impact fair valued securities and for discussing and assessing fair values on an ongoing basis, and at least quarterly, with the Valuation Committee of the Board of Trustees.

Inputs used when applying fair value methods to value a security may include credit ratings, the financial condition of the company, current market conditions and comparable securities. The Portfolio may use fair value methods if it is determined that a significant event has occurred after the close of the exchange or market on which the security trades and prior to the determination of the Portfolio’s net asset value. Examples of a significant event might include political or economic news, corporate restructurings, natural disasters, terrorist activity or trading halts. Thus, the valuation of the Portfolio’s securities may differ significantly from exchange prices, and such differences could be material.

At August 31, 2021, no securities were valued using fair value methods (other than securities valued using prices supplied by independent pricing services, broker-dealers or using a third party insurance industry pricing model).

Interest income, including interest on income-bearing cash accounts, is recorded on the accrual basis. Dividend and interest income are reported net of unrecoverable foreign taxes withheld at the applicable country rates and net of income accrued on defaulted securities.

Interest and dividend income payable by delivery of additional shares is reclassified as PIK (payment-in-kind) income upon receipt and is included in interest and dividend income, respectively.

Security transactions are recorded as of trade date. Gains and losses on sales of investments are calculated on the identified cost method for both financial reporting and federal income tax purposes.

The Portfolio makes a daily allocation of its net investment income and realized and unrealized gains and losses from securities, futures and foreign currency transactions to its investors in proportion to their investment in the Portfolio.

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The Portfolio is classified as a partnership for federal income tax purposes. As such, each investor in the Portfolio is treated as owner of its proportionate share of the net assets, income, expenses and realized and unrealized gains and losses of the Portfolio. Therefore, no federal income tax provision is required. It is intended that the Portfolio’s assets will be managed so an investor in the Portfolio can satisfy the requirements of Subchapter M of the Internal Revenue Code.

Management has analyzed the Portfolio’s tax positions taken on income tax returns for all open tax periods and has concluded that no provision for income tax is required in the Portfolio’s financial statements. For the period ended August 31, 2021, the Advisor has analyzed the Portfolio’s inventory of tax positions taken on with respect to all applicable income tax issues for all open tax periods (in each respective jurisdiction), and has concluded that no provision for income tax is required in the Portfolio's financial statements.

The value of securities held by the Portfolio may go up or down, sometimes rapidly or unpredictably, due to general market conditions, such as real or perceived adverse economic, political or regulatory conditions, recessions, the spread of infectious illness or other public health issues, inflation, changes in interest rates, lack of liquidity in the bond markets or adverse investor sentiment. In the past several years, financial markets have experienced increased volatility, depressed valuations, decreased liquidity and heightened uncertainty. These conditions may continue, recur, worsen or spread. A general rise in interest rates could adversely affect the price and liquidity of fixed-income securities and could also result in increased redemptions from the Portfolio.

At times, the Portfolio’s investments may represent industries or industry sectors that are interrelated or have common risks, making the Portfolio more susceptible to any economic, political, or regulatory developments or other risks affecting those industries and sectors. The Portfolio’s investments in foreign markets and countries with limited developing markets may subject the Portfolio to a greater degree of risk than investments in a developed market. These risks include disruptive political or economic conditions and the imposition of adverse governmental laws or currency exchange restrictions.

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The Portfolio invests in below-investment-grade (high-yield) debt securities and preferred stocks. Some of these high-yield securities may be convertible into equity securities of the issuer. Debt securities rated below investment-grade are commonly referred to as “junk bonds” and are considered speculative. These securities involve greater risk of loss, are subject to greater price volatility, and are less liquid, especially during periods of economic uncertainty or change, than higher rated debt securities.

The municipal bond market can be susceptible to unusual volatility, particularly for lower-rated and unrated securities. Liquidity can be reduced unpredictably in response to overall economic conditions or credit tightening. Municipal issuers may be adversely affected by rising health care costs, increasing unfunded pension liabilities, and by the phasing out of federal programs providing financial support. Unfavorable conditions and developments relating to projects financed with municipal securities can result in lower revenues to issuers of municipal securities, potentially resulting in defaults. Issuers often depend on revenues from these projects to make principal and interest payments. The value of municipal securities can also be adversely affected by changes in the financial condition of one or more individual municipal issuers or insurers of municipal issuers, regulatory and political developments, tax law changes or other legislative actions, and by uncertainties and public perceptions concerning these and other factors. Municipal securities may be more susceptible to downgrades or defaults during recessions or similar periods of economic stress.

Financial difficulties of municipal issuers may continue or get worse. To the extent the Portfolio invests significantly in a single state, including California, Illinois, New York and Indiana, or in securities the payments on which are dependent upon a single project or source of revenues, or that relate to a sector or industry, including health care facilities, education, transportation, special revenues and pollution control, the Portfolio will be more susceptible to associated risks and developments.

The Portfolio’s investments, payment obligations and financing terms may be based on floating rates, such as LIBOR (London Interbank Offered Rate). Plans are underway to phase out the use of LIBOR. The UK Financial Conduct Authority (“FCA”) and LIBOR’s administrator, ICE Benchmark Administration (“IBA”), have announced that most LIBOR rates will no longer be published after the end of 2021 and a majority of U.S. dollar LIBOR rates will no longer be published after June 30, 2023. It is possible that the FCA may compel the IBA to publish a subset of LIBOR settings after these dates on a “synthetic” basis, but any such publications would be considered non-representative of the underlying markets. Actions by regulators have resulted in the establishment of alternative reference rates to LIBOR in most major currencies. Based on the recommendations of the

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New York Federal Reserve’s Alternative Reference Rate Committee (comprised of major derivative market participants and their regulators), the U.S. Federal Reserve began publishing a Secured Overnight Funding Rate (“SOFR”) that is intended to replace U.S. Dollar LIBOR. Proposals for alternative reference rates for other currencies have also been announced or have already begun publication, such as SONIA in the United Kingdom. Markets are slowly developing in response to these new rates, and transition planning is at a relatively early stage. Neither the effect of the transition process nor its ultimate success is known. The transition process may lead to increased volatility and illiquidity in markets that currently rely on LIBOR to determine interest rates. The effect of any changes to — or discontinuation of — LIBOR on the portfolio will vary depending on, among other things, provisions in individual contracts and whether, how, and when industry participants develop and adopt new reference rates and alternative reference rates for both legacy and new products and instruments. Because the usefulness of LIBOR as a benchmark may deteriorate during the transition period, these effects could materialize prior to the end of 2021.

With the increased use of technologies such as the Internet to conduct business, the Portfolio is susceptible to operational, information security and related risks. While the Portfolio’s Adviser has established business continuity plans in the event of, and risk management systems to prevent, limit or mitigate, such cyber-attacks, there are inherent limitations in such plans and systems, including the possibility that certain risks have not been identified. Furthermore, the Portfolio cannot control the cybersecurity plans and systems put in place by service providers to the Portfolio such as Brown Brothers Harriman & Co., the Portfolio’s custodian and accounting agent, and DST Asset Manager Solutions, Inc., the Portfolio’s transfer agent. In addition, many beneficial owners of Portfolio shares hold them through accounts at broker-dealers, retirement platforms and other financial market participants over which neither the Portfolio nor the Adviser exercises control. Each of these may in turn rely on service providers to them, which are also subject to the risk of cyber-attacks. Cybersecurity failures or breaches at the Adviser or the Portfolio’s service providers or intermediaries have the ability to cause disruptions and impact business operations, potentially resulting in financial losses, interference with the Portfolio’s ability to calculate its net asset value, impediments to trading, the inability of Portfolio investors to effect contributions or withdrawals from the Portfolio or receive distributions, loss of or unauthorized access to private shareowner information and violations of applicable privacy and other laws, regulatory fines, penalties, reputational damage, or additional compliance costs. Such costs and losses may not be

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covered under any insurance. In addition, maintaining vigilance against cyber-attacks may involve substantial costs over time, and system enhancements may themselves be subject to cyber-attacks.

COVID-19

The global pandemic of the novel coronavirus respiratory disease designated COVID-19 has resulted in major disruption to economies and markets around the world, including the United States. Global financial markets have experienced extreme volatility and severe losses, and trading in many instruments has been disrupted. Liquidity for many instruments has been greatly reduced for periods of time. Some interest rates are very low and in some cases yields are negative. Some sectors of the economy and individual issuers have experienced particularly large losses. Rates of inflation have recently risen. These circumstances may continue for an extended period of time, and may continue to affect adversely the value and liquidity of the Portfolio’s investments. Governments and central banks, including the Federal Reserve in the U.S., have taken extraordinary and unprecedented actions to support local and global economies and the financial markets. These actions have resulted in significant expansion of public debt, including in the U.S. The impact of these measures will not be known for some time. The consequences of high public debt, including its future impact on the economy and securities markets, likewise may not be known for some time.

The Portfolio’s registration statement on Form N-1A contains unaudited information regarding the Portfolio’s principal risks. Please refer to that document when considering the Portfolio’s principal risks.

Restricted Securities are subject to legal or contractual restrictions on resale. Restricted securities generally are resold in transactions exempt from registration under the Securities Act of 1933. Private placement securities are generally considered to be restricted except for those securities traded between qualified institutional investors under the provisions of Rule 144A of the Securities Act of 1933.

Disposal of restricted investments may involve negotiations and expenses, and prompt sale at an acceptable price may be difficult to achieve. Restricted investments held by the Portfolio at August 31, 2021 are listed in the Schedule of Investments.

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2. Management Agreement

The Adviser manages the Portfolio’s portfolio. The Portfolio does not pay a management fee under the Portfolio’s investment advisory agreement with the Adviser.

3. Compensation of Trustees and Officers

The Portfolio pays an annual fee to its Trustees. The Adviser reimburses the Portfolio for fees paid to the Interested Trustees. The Portfolio does not pay any salary or other compensation to its officers. For the period from December 21, 2020 (commencement of operations) through August 31, 2021, the Portfolio paid $36,100 in Trustees’ compensation, which is reflected on the Statement of Operations as Trustees’ fees. At August 31, 2021, the Portfolio had a payable for Trustees’ fees on its Statement of Assets and Liabilities of $14,470.

4. Transfer Agent

Brown Brothers Harriman & Co. serves as the transfer agent to the Portfolio at negotiated rates. Transfer agent fees and payables are shown on the Statement of Operations and the Statement of Assets and Liabilities, respectively.

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Report of Independent Registered Public Accounting Firm

To the Board of Trustees of Pioneer Core Trust I and the Shareowners of Pioneer High Income Municipal Portfolio:

Opinion on the Financial Statements

We have audited the accompanying statement of assets and liabilities of Pioneer High Income Municipal Portfolio (the “Fund”) (one of the funds constituting Pioneer Core Trust I (the “Trust”)), including the schedule of investments, as of August 31, 2021, and the related statement of operations, the statement of changes in net assets and the financial highlights for the period from December 21, 2020 (commencement of operations) through August 31, 2021 and the related notes (collectively referred to as the “financial statements”). In our opinion, the financial statements present fairly, in all material respects, the financial position of Pioneer High Income Municipal Portfolio (one of the funds constituting Pioneer Core Trust I) at August 31, 2021, and the results of its operations, the changes in its net assets and its financial highlights for the period from December 21, 2020 (commencement of operations) through August 31, 2021, in conformity with U.S. generally accepted accounting principles.

Basis for Opinion

These financial statements are the responsibility of the Trust’s management. Our responsibility is to express an opinion on the Fund’s financial statements based on our audit. We are a public accounting firm registered with the Public Company Accounting Oversight Board (United States) (“PCAOB”) and are required to be independent with respect to the Company in accordance with the U.S. federal securities law and the applicable rules and regulations of the Securities and Exchange Commission and the PCAOB.

We conducted our audit in accordance with the standards of the PCAOB. Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements are free of material misstatement, whether due to error or fraud. The Trust is not required to have, nor were we engaged to perform, an audit of the Trust’s internal control over financial reporting. As part of our audit, we are required to obtain an understanding of internal control over financial reporting, but not for the purpose of expressing an opinion on the effectiveness of the Trust’s internal control over financial reporting. Accordingly, we express no such opinion.

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Our audit included performing procedures to assess the risks of material misstatement of the financial statements, whether due to error or fraud, and performing procedures that respond to those risks. Such procedures included examining, on a test basis, evidence regarding the amounts and disclosures in the financial statements. Our procedures included confirmation of securities owned as of August 31, 2021 by correspondence with the custodian and others or by other appropriate auditing procedures where replies from others were not received. Our audit also included evaluating the accounting principles used and significant estimates made by management, as well as evaluating the overall presentation of the financial statements. We believe that our audit provides a reasonable basis for our opinion.

We have served as the auditor of one or more investment companies in the Pioneer family of funds since 2017.

Boston, Massachusetts
November 4, 2021

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Statement Regarding Liquidity Risk Management Program

As required by law, the Portfolio has adopted and implemented a liquidity risk management program (the “Program”) that is designed to assess and manage liquidity risk. Liquidity risk is the risk that the Portfolio could not meet requests to effect withdrawals from the Portfolio without significant dilution of remaining investors’ interests in the Portfolio. The Portfolio’s Board of Trustees designated a liquidity risk management committee (the “Committee”) consisting of employees of Amundi Asset Management US, Inc. (the “Adviser”) to administer the Program.

The Committee provided the Board of Trustees with a report that addressed the operation of the Program and assessed its adequacy and effectiveness of implementation (the “Report”). The Report covered the period from December 21, 2020 (commencement of operations) through December 31, 2020 (the “Reporting Period”).

The Report confirmed that, throughout the Reporting Period, the Committee had monitored the Portfolio’s portfolio liquidity and liquidity risk on an ongoing basis, as described in the Program and in Board reporting throughout the Reporting Period.

The Report discussed the Committee’s annual review of the Program, which addressed, among other things, the following elements of the Program:

The Committee reviewed the Portfolio’s investment strategy and liquidity of portfolio investments during both normal and reasonably foreseeable stressed conditions. The Committee noted that the Portfolio’s investment strategy continues to be appropriate for an open-end fund, taking into account, among other things, whether and to what extent the Portfolio held less liquid and illiquid assets and the extent to which any such investments affected the Portfolio’s ability to meet withdrawal requests. In managing and reviewing the Portfolio’s liquidity risk, the Committee also considered the extent to which the Portfolio’s investment strategy involves a relatively concentrated portfolio or large positions in particular issuers, the extent to which the Portfolio uses borrowing for investment purposes, and the extent to which the Portfolio uses derivatives (including for hedging purposes). The Committee also reviewed the Portfolio’s short-term and long-term cash flow projections during both normal and reasonably foreseeable stressed conditions. In assessing the Portfolio’s cash flow projections, the Committee considered, among other factors, historical net withdrawal activity, withdrawal policies, ownership concentration, and the degree of certainty associated with the Portfolio’s short-term and long-term cash flow projections. The Committee also considered the Portfolio’s holdings of cash

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and cash equivalents, as well as borrowing arrangements and other funding sources, including, if applicable, the Portfolio’s participation in a credit facility, as components of the Portfolio’s ability to meet withdrawal requests. The Portfolio has adopted an in-kind withdrawal policy which may be utilized to meet larger redemption requests.

The Committee reviewed the Program’s liquidity classification methodology for categorizing the Portfolio’s investments into one of four liquidity buckets. In reviewing the Portfolio’s investments, the Committee considered, among other factors, whether trading varying portions of a position in a particular portfolio investment or asset class in sizes the Portfolio would reasonably anticipate trading, would be reasonably expected to significantly affect liquidity.

The Committee performed an analysis to determine whether the Portfolio is required to maintain a Highly Liquid Investment Minimum, and determined that no such minimum is required because the Portfolio primarily holds highly liquid investments.

The Report stated that the Committee concluded the Program operates adequately and effectively, in all material respects, to assess and manage the Portfolio’s liquidity risk throughout the Reporting Period.

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Trustees, Officers and Service Providers

Investment Adviser and Administrator
Amundi Asset Management US, Inc.

Custodian and Sub-Administrator
Brown Brothers Harriman & Co.

Independent Registered Public Accounting Firm
Ernst & Young LLP

Principal Underwriter/Placement Agent
Amundi Distributor US, Inc.

Legal Counsel
Morgan, Lewis & Bockius LLP

Transfer Agent
DST Asset Manager Solutions, Inc.

Proxy Voting Policies and Procedures of the Fund/Portfolio are available without charge, upon request, by calling our toll free number (1-800-225-6292). Information regarding how the Fund/Portfolio voted proxies relating to portfolio securities during the most recent 12-month period ended June 30 is publicly available to shareowners at www.amundi.com/us. This information is also available on the Securities and Exchange Commission’s web site at www.sec.gov.

Trustees and Officers

The Fund’s and Portfolio’s Trustees and officers are listed below, together with their principal occupations and other directorships they have held during at least the past five years. Trustees who are interested persons of the Fund and Portfolio within the meaning of the 1940 Act are referred to as Interested Trustees. Trustees who are not interested persons of the Fund and Portfolio are referred to as Independent Trustees. Each of the Trustees serves as a Trustee of each of the 49 U.S. registered investment portfolios for which Amundi US serves as investment adviser (the “Pioneer Funds”). The address for all Trustees and all officers of the Fund is 60 State Street, Boston, Massachusetts 02109.

The Statement of Additional Information of the Fund includes additional information about the Trustees and is available, without charge, upon request, by calling 1-800-225-6292.

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Independent Trustees

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Independent Trustees (continued)

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Interested Trustees

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Fund Officers

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Fund Officers (continued)

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How to Contact Amundi

We are pleased to offer a variety of convenient ways for you to contact us for assistance or information.

Write to us:

Amundi
P.O. Box 219427
Kansas City, MO 64121-9427

This report must be preceded or accompanied by a prospectus.

Each of the Fund and Portfolio files a complete schedule of portfolio holdings with the Securities and Exchange Commission for the first and third quarters of each fiscal year as an exhibit to its reports on Form N-PORT. Shareholders may view the filed Form N-PORT by visiting the Commission’s web site at https://www.sec.gov.

Amundi Asset Management US, Inc.
60 State Street
Boston, MA 02109
www. amundi. com/us

Securities offered through Amundi Distributor US, Inc.,
60 State Street, Boston, MA 02109
Underwriter of Pioneer Mutual Funds, Member SIPC
© 2021 Amundi Asset Management US, Inc. 32682-00-1021