Line 12b is for "charitable contributions if you take the standard deduction", so I left it blank because I am itemizing deductions, not using the standardized deduction. I also tried entering zero. Currently, the amount in 12a is also in 12c, which is correct. The sum of 12a and 12b should be entered in 12c, which is why 12b should be blank or zero. Turbo Tax continues flagging this in the review process and wants me to enter something in that field, but what??
