There is no box 5a on a Form 1099-DIV. Do you mean box 5? The IRS instructions for Form 1099-DIV say that the amount in box 5 must be included in the amount in box 1a. So a 1099-DIV with an amount in box 5 and nothing in box 1a is not filled out correctly and is not valid. Whoever confirmed that your 1099-DIV is "accurate" obviously did not read the IRS instructions. Contact the issuer of the 1099-DIV to get a corrected Form 1099-DIV.
The following is from the IRS Instructions for Form 1099-DIV, on page 5.
"Box 5. Section 199A Dividends
Enter the qualified REIT dividends paid by a REIT or section 199A dividends paid by a RIC to the recipient. This amount is included in the amount reported in box 1a."
