There should be supporting statements with your K-1, if none of them mentions U.S. debt, don’t worry about it. Regardless, the only reason this amount is there at all is for state income tax purposes. It could potentially be a deduction on your state return. Typically us debt obligations are state income tax exempt, but are not Federal Tax Exempt. So when listing your dividends or interest on a state return they wouldn’t be included or they would be deducted.