Interest income on securities increased $19,000 to $144,000 for the three months ended December 31, 2018 from $125,000 for the three months ended December 31, 2017. Our average balance of securities increased $586,000, to $22.1 million for the three months ended December 31, 2018 from $21.5 million for the three months ended December 31, 2017, and the average yield on securities increased to 2.43% from 2.24%.
Interest income on interest-earning deposits increased $103,000 to $200,000 for the three months ended December 31, 2018 from $97,000 for the three months ended December 31, 2017. The increase in interest income on interest-earning deposits was due to a 170 basis point increase in yield, reflecting increases in market interest rates. Our average balance of interest-earning deposits decreased $2.0 million, or 7.2%, to $25.9 million for the three months ended December 31, 2018 from $27.9 million for the three months ended December 31, 2017, as we deployed excess cash and cash equivalents into higher yielding investments.
Interest Expense. Interest expense increased $166,000, or 48.8%, to $506,000 for the three months ended December 31, 2018 compared to $340,000 for the three months ended December 31, 2017, due to increases in interest expense on deposits and borrowings. We experienced an increase in interest expense for all deposit categories. Specifically, interest expense on certificates of deposit increased $72,000, or 30.0%, to $312,000 for the three months ended December 31, 2018 from $240,000 for the three months ended December 31, 2017. The average rate we paid on certificates of deposit increased 33 basis points to 1.45% for the three months ended December 31, 2018 from 1.12% for the three months ended December 31, 2017, reflecting our increasing interest rates in response to changes in market interest rates. The average balance of certificates of deposit increased to $86.2 million for the three months ended December 31, 2018 compared to $85.4 million for the three months ended December 31, 2017. Interest expense on interest-bearing checking accounts increased $43,000 to $102,000 for the three months ended December 31, 2018 compared to $59,000 for the three months ended December 31, 2017. The average balance of and rate paid on interest-bearing checking accounts increased to $53.9 million and 0.76%, respectively, for the three months ended December 31, 2018, from $39.4 million and 0.60%, respectively, for the three months ended December 31, 2017, as we continue to see an increase in the number of Kasasa reward based checking accounts opened.
Interest expense on borrowings increased to $38,000 for the three months ended December 31, 2018 compared to $24,000 for the three months ended December 31, 2017, reflecting borrowing from the Federal Home Loan Bank of Atlanta to fund loan growth and support operations.
Net Interest Income. Net interest income increased $98,000, or 3.3%, to $3.1 million for the three months ended December 31, 2018 from $3.0 million for the three months ended December 31, 2017, as our interest income increased faster than our internal interest expense. Our average net interest-earning assets decreased by $10.1 million, or 11.5%, to $77.5 million for the three months ended December 31, 2018 from $87.5 million for the three months ended December 31, 2017, and our net interest rate spread decreased by seven basis points to 4.22% for the three months ended December 31, 2018 from 4.29% for the three months ended December 31, 2017, reflecting increases in the average balance of interest-bearing liabilities and the rate paid on interest-bearing liabilities. Our net interest margin decreased by four basis points to 4.51% for the three months ended December 31, 2018 from 4.55% for the three months ended December 31, 2017.
Provision for Loan Losses. We recorded no provision for loan losses for the three months ended December 31, 2018 or the three months ended December 31, 2017. Our allowance for loan losses was $4.0 million at December 31, 2018 compared to $3.9 million at September 30, 2018 and $4.6 million at December 31, 2017. The allowance for loan losses to total loans was 1.74% at December 31, 2018 compared to 1.73% at September 30, 2018 and 2.09% at December 31, 2017. The allowance for loan losses to non-performing loans was 220.0% at December 31, 2018 compared to 178.10% at September 30, 2018 and 94.95% at December 31, 2017. We had net recoveries of $113,000 during the three months ended December 31, 2018.
To the best of our knowledge, we have recorded all loan losses that are both probable and reasonable to estimate at December 31, 2018. However, future changes in the factors we use to calculate the allowance for loan losses, including, but not limited to, actual loss experience with respect to our loan portfolio, could result in material increases in our provision for loan losses. In addition, the Office of the Comptroller of the Currency, as an integral
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