I had exactly the same problem. This is a fairly uncommon tax situation so I think it has not gotten much attention. The "Can't be above zero" error message that appeared for line D of the "Earlier Year Tax Return Smart Worksheet" appears to be a programming error on a different worksheet that is causing the error message.
I had loaded all data in TurboTax using step-by-step data entry.
After reading the worksheet instructions line by line I realized there is an error in the "Earlier Year Lump-Sum Social Security Worksheet" Line B. Line B is supposed to be any SS benefits received in the earlier year being calculated. Even though the correct value was loaded in the Step-by-Step, Line B was calculated at 0 (zero). When I used form OVERRIDE and entered the correct value, the "Can't be above zero" error message that appeared for line D of the "Earlier Year Tax Return Smart Worksheet" disappeared.
