IRS request for 1040X copy after late filed 8606. Thank you!

Hello, and happy holidays.  A question, please.

 

In March 2019 I made a nondeductible contribution to a traditional IRA for the 2018 tax year.  In March 2020 I realized that I had failed to file form 8606, so I late filed it that very week.  I included a letter asking for a penalty waiver since I was taking "immediate corrective action."

 

This week I received a letter from the IRS asking for “a completed copy of Form 1040X, Amended US Individual Income Tax Return, page 1, column C.”

 

I hadn’t filed a 2018 1040X because it didn’t seem to me like any amendments were necessary.  That is, my failure to timely file 8606 seemed to me to be just a reporting mistake.  Please, am I right?  And if so, what should my letter I send back to the IRS say?

 

Thank you most sincerely for your help.