Negative Number and form 6781

[rsherry8]

I am short puts and calls on the S&P 500 index. These are 1256 based options. As such, I need to mark to the market using form 6781. Since, I sold to open the position, I consider my basis to be negative. However, TurboTax is generating an error for a negative basis. In particular, the problem is box 14C.

Is this a bug in TurboTax? What should I do?