Retirement RMD and Roth conversion

I have been taking RMD for a few years.  This year I only took 1/2 as a normal distribution.  Because I didn't need the money, I took the balance of the required amount, plus a little bit more, and converted it to my Roth.  Taxes are withheld from the withdrawals.  Turbo Tax is checking my entry and says "partial Roth conversion amount cannot be greater than the amount on line B1......."  Line B1 is a check box - no dollar amount is identified.  I have put the part that was converted on B2 and the same number appears in B6 in red and I get an error message.  How do I proceed?