Form 1099-R for a regular distribution from a traditional IRA must never have a zero in box 2a. It is required to show the same amount in box 2a as is in box 1. The Form 1099-R for an IRA distribution must be entered exactly as shown on the form received from the payer. If you haven't yet received the Form 1099-R from the payer, delete whatever you have entered regarding this distribution and wait for the form to arrive from the payer.
