Deductions & credits

If I make my “Outstanding Mortgage Principle” (Box 2 1098) $0 because I paid it off, I get an error when I try to file and TurboTax asked me to put in a figure.  I am confused why the 1098 mortgage refinance instructions say this:

  1. Gather all of your 1098 forms related to your refinance, e.g. the form from your original lender and the form from your new lender.
  2. Grab a calculator and add the amount in Box 1 Mortgage Interest from each form and enter the total here as Box 1 Mortgage Interest.