Presumably you are talking about a tax return for some year prior to 2018, perhaps 2017. Form 1040 line 8a is for interest income, generally reported on a Form 1099-INT, not for retirement income that would have been reported on a Form 1099-R. Taxable income reported on a Form 1099-R should generally be present on line 15b or 16b. Perhaps you mistakenly entered a Form 1099-R as a Form 1099-INT. The taxable result will not necessarily be the same, but it might be. I would prepare a corrected tax return to determine if there is any difference in tax liability as compared to your originally filed tax return, then contact the IRS as indicated in the letter to find out how they would like you to report the correction. They will probably ask you to submit the amended tax return.
