Yes you are correct there is a question on the N-400 that asked "Have you EVER claimed to be a U.S. citizen (in writing or any other way")? Part 12 question 1, it also ask for an explanation on a separate sheet of paper if you answered yes, so you'd have to explain it there. I'd be proactive now and in addition to amending your taxes I would get a affidavit from your tax preparer stating what happened and include that with your N-400 documents. Not sure if it will be an issue, but one thing we do know it will be an issue if you fail to tell the truth and they find out otherwise. Your proactive steps taken now I think will save you heartache when the time comes.