In this case with box 5 equaling box 1 and box 2b Taxable amount not determined unmarked, the blank box 2a means $0 is taxable. The payer should have put a zero in box 2a, but some payers have faulty systems for generating Forms 1099-R. If TurboTax complains about the blank box 2a or includes any of this money in income on Form 1040 line 4d, enter a zero in box 2a of TurboTax's 1099-R form. (Since your AGI does not change when you enter this Form 1099-R, it seems that TurboTax is handling it correctly.)
That was a perfect answer to my situation even though my specifics were a bit diff. As soon as I entered a zero in 2b, bam! All my numbers self corrected.
I was dealing with a 1099r rollover that TT wanted to include as income. That zero adjusted it perfectly.
Thanks!