1099-R Qualified Pension gross distribution not showing up in 1040 Line 5a

I have a 1099-R for a qualified pension that I started receiving monthly payments for this year. It is all taxable and I am well over retirement age. I would expect the amount in Box 1 of the 1099-R (Gross distribution) to show up on Line 5a Form 1040 (Pensions, and annuities), but it remains blank. However, Line 5b (Taxable amount) shows that number.

 

As a result, the income and tax are calculated correctly, but I'm curious what Line 5a is blank.