An insurer may invest in bonds, notes, or evidences of indebtedness of any municipal or county utility within the United States or Canada, which are payable from revenues or earnings specifically pledged for the payment of the principal and interest on the obligations, and for the payment of which a lawful sinking fund or reserve fund has been established and is being maintained, but only if no default in payment of principal or interest on the obligations to be purchased has occurred within five (5) years of the investment.