Mortgage paid off, what is the "final principal balance"?

If you paid off your mortgage in 2018, what do you put for "final principal balance"? The TurboTax page says "If you paid off your loan this year, we need the balance before you made your final payment.", however, the answer by the TurboTax TaxPro on this question (https://intuit.me/2TAHzTc) says to put $0. Which is right?