The only way I've been able to come up with line 5b being more than line 5a is to enter an amount in box 2a of a 1099R that is more than the amount that is in box 1. That seems to be opposite what you described in the original question, but that's the only reason I know of that TurboTax would flag the entry in box 2a.
I would delete any erroneously entered Forms 1099-R (the code-G Form 1099-R) and reenter them.
