An insurer may invest any of its funds in bonds or other evidences of debt, not in default as to principal or interest, which are valid and legally authorized obligations issued, assumed or guaranteed by the United States or by any jurisdiction of the United States or by any political subdivision thereof or by any civil division or public instrumentality of one or more of the foregoing, if, by statutory or other legal requirements applicable thereto, such obligations are payable, as to both principal and interest, (1) from taxes levied or required to be levied upon all taxable property or all taxable income within the jurisdiction of such governmental unit or, (2) from adequate special revenues pledged or otherwise appropriated or by law required to be provided for the purpose of such payment, but not including any obligation payable solely out of special assessments on properties benefited by local improvements unless adequate security is evidenced by the ratio of assessment to the value of the property or the obligation is additionally secured by an adequate guaranty fund required by law.