(1) Except as provided in subsection (2), overpaid benefits and unpaid contributions accrue interest at the retirement system's actuarially assumed annual rate of return, compounded monthly. Interest accrues beginning on the date that the first erroneous payment was made or that the contributions were first due and continues to accrue until the amount owed to the retirement system is fully paid.
(2) If overpaid benefits or unpaid contributions resulted solely from an error made by the retirement system, the amount owed may not include interest.