An insurer may invest in bonds or other evidences of indebtedness, 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 state thereof, or by Canada or any of the provinces thereof, or by any county, city, town, village, municipality or district therein or by any political subdivision thereof or by a 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, from taxes levied or required to be levied upon all taxable property or all taxable income within the jurisdiction of such governmental unit, or from adequate special revenues pledged or otherwise appropriated or by law required to be provided for the purpose of such payment.