Error on Form 1116

When I do Smart Check, I get an error on Form 1116 Comp Wks, Part 1 Taxable income (loss), line 4 Interest expense, b Deductible other interest, (1) c Adjusted basis of total investment assets. I do not have any interest expense; Line 4b(1)a is ZERO. All of my foreign income is from mutual funds. TurboTax will not accept a zero here. I always have Form 1116 and never had to enter this. Is this a bug?

Just to move forward, I entered a "1". The error is cleared and all looks correct. What is this about anyway?