I have both deductible and nondeductible retirement accounts. Do I need to add nondeductible cost basis if I converted from a pretax account?

I have both deductible and nondeductible retirement accounts.  I did a Roth conversion from my pretax account.  TurboTax asks my cost basis for my nondeductible account even though the entire conversion was from my pretax account.  It then calculated that part of my conversion is not taxable.  Is that correct?  Why would my nondeductible IRA affect how much of the conversion from my pretax account is taxable?