26 U.S.C. § 4373: Exemptions

The tax imposed by section 4371 shall not apply to—(1) Effectively connected itemsAny amount which is effectively connected with the conduct of a trade or business within the United States unless such amount is exempt from the application of section 882(a) pursuant to a treaty obligation of the United States. (2) Indemnity bondAny indemnity bond required to be filed by any person to secure payment of any pension, allowance, allotment, relief, or insurance by the United States, or to secure a duplicate for, or the payment of, any bond, note, certificate of indebtedness, war-saving certificate, warrant or check, issued by the United States.