An insurer may invest in bonds, notes, warrants and other securities not in default which are the direct obligations of any state of the United States or of the District of Columbia, or of the government of Canada or any province thereof, or for which the full faith and credit of such state, district, government or province has been pledged for the payment of principal and interest. Bonds, notes, warrants and other securities classified as revenue, prerefunded or declining balances are not considered acceptable investments for this purpose.