An insurer may invest in bonds or other evidences of indebtedness which are general obligations of or are adequately secured as to both principal and interest by irrevocable pledge of specific revenues by this state or any other state of the United States or any county, incorporated city or town, or duly organized school district or other civil division, governmental unit, or public instrumentality of any such state. Obligations payable solely out of special assessments on properties benefited by local improvements shall not be eligible under this section.
Ala. Code § 27-41-9 (1975)