Examine Form 1040 lines 4a and 4b. If you see less added to line 4b than the amount you are entering as your Roth conversion, it implies that you've told TurboTax that you have basis in nondeductible traditional IRA contributions. Also make sure that you are not indicating some amount as having been rolled over instead of converted to Roth. Examine Form 8606, the basis shown on the IRA Information Worksheet and the details in section B of TurboTax's 1099-R form.
