Enter two Form 1099-Rs for the same payer, one with the amount received before you reached Minimum Retirement Age (use Distribution Code 3) and the other with the amount you received after reaching MRA (use Distribution Code 7).
Make sure the amounts in boxes 1, 2 and 4 of each form add up to the totals in the Form 1099-R you received. If there were any state or local amounts reported, you can probably report them all on one of the "split-1099-Rs"). This work-around should not have any effect on your tax calculation or create a mismatch with the IRS record of the 1099-R - they are concerned with the total amount paid to you, not whether it is split between two 1099-Rs.
