The average balance of investment securities increased $16.9 million, or 64.2%, to $43.3 million for the three months ended December 31, 2022, from December 31, 2021, while the average yield on investment securities increased by 174 basis points to 2.92% for the three months ended December 31, 2022, from 1.18% for the three months ended December 31, 2021. This increase in yields resulted from the effects of management’s sale of lower yielding investments in December 2021 and purchases of higher yielding securities during the quarter ended September 30, 2022.
The average balance of other interest-bearing deposits, comprised of certificates of deposit in other financial institutions, overnight deposits and stock in the Federal Home Loan Bank, decreased $5.4 million, or 23.7%, for the three months ended December 31, 2022, and the average yield increased 344 basis points to 3.88% for the three months ended December 31, 2022, from 0.44% for the three months ended December 31, 2021 reflecting the rise in the interest rate environment.
Interest Expense. Total interest expense increased $1,000, or 0.6%, to $85,000 for the three months ended December 31, 2022, compared to $84,000 for the three months ended December 31, 2021. The increase was primarily due to a increase in the average cost of deposits to 0.33% for the three months ended December 31, 2022, from 0.32% for the three months ended December 31, 2021, reflecting how management has worked to manage the cost of deposits over the period as interest rates in the economy have been increasing in recent months, which was partially offset by an increase of $304,000, or 0.3%, in the average balance of deposits, to $104.6 million for the three months ended December 31, 2022, compared to $104.3 million for three months ended December 31, 2021.
Net Interest Income. Net interest income increased $425,000, or 60.6%, to $1.1 million for the three months ended December 31, 2022, compared to $701,000 for the three months ended December 31, 2021. The increase reflected an increase in the interest rate spread to 3.03% for the three months ended December 31, 2022, from 2.13% for the three months ended December 31, 2021. The net interest margin increased to 3.12% for the three months ended December 31, 2022, from 2.19% for the three months ended December 31, 2021.
Provision for Loan Losses. Based on an analysis of the factors described in “Critical Accounting Policies and Use of Critical Accounting Estimates – Allowance for Loan Losses,” management concluded that a provision for loan losses was not required for each of the three months ended December 31, 2022 and 2021. The allowance for loan losses was $223,000 at both December 31, 2022 and 2021 and represented 0.28% of total loans at December 31, 2022, and 0.29% of total loans at December 31, 2021. The determination over the adequacy of the allowance for loan losses was due primarily to the low balances of nonperforming loans, delinquent loans and no net charge-offs in both periods.
Total nonperforming loans were $267,000 at December 31, 2022, compared to $161,000 at December 31, 2021. Total loans past due greater than 30 days were $1.3 million and $1.5 million at those respective dates. As a percentage of nonperforming loans, the allowance for loan losses was 83.5% at December 31, 2022, compared to 138.2% at December 31, 2021.
The allowance for loan losses reflects the estimate management believes to be appropriate to cover incurred probable losses which were inherent in the loan portfolio at December 31, 2022 and 2021. While management believes the estimates and assumptions used in the determination of the adequacy of the allowance are reasonable, such estimates and assumptions could be proven incorrect in the future, and the actual amount of future provisions may exceed the amount of past provisions. Furthermore, as an integral part of its examination process, the OCC will periodically review our allowance for loan losses. The OCC may have judgments different than those of management, and we may determine to increase our allowance as a result of these regulatory reviews. Any material increase in the allowance for loan losses may adversely affect our financial condition and results of operations.
Non-Interest Income. Non-interest income decreased by $3,000, or 5.3%, to $52,000 for the three months ended December 31, 2022, compared to $55,000 for the three months ended December 31, 2021 due to normal fluctuations in the volume of fees on loans and deposits.
Noninterest Expense. Noninterest expense increased $835,000, or 85.9%, to $1.8 million for the three months ended December 31, 2022, compared to $1.0 million for the three months ended December 31, 2021. The increase reflects $56,000, or 16.1%, increase in salaries and employee benefits, a $147,000, or 430.0%, increase in professional services and a decrease of $291,000, or 100%, of loss on the sale of securities compared to the three month period ended December 31, 2021. The increase in professional services was due primarily to costs related to increased costs of operating and the reporting requirements of a public stock company. There was also a one-time charge of $930,000 during the period associated with withdrawing from the multiemployer plan.