If your return requires you to report your IRA basis, enter the correct amount. The same basis doesn't have to carry over from year to year, if a change occurred during the year.
Check the amount you entered for IRA Contributions in your 2018 return. You may need to change this.
Click this link for info on changing the basis on Form 8606 that may help you.
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Thank you so much. I did download my 2018 return and it shows the basis (incorrectly) as $6,500 (line 14 of 8606) but also shows there was no taxable amount. I think I may only need to correct the basis in the 2020 return, and not worry about amending the 2018 return since it resulted in no tax liability. Do you agree with that approach?
Yes, your approach is correct. According to the IRS, you do not have to amend a tax return when the result would be inconsequential.
Here is the IRS language on their website:
If you discover an error after filing your return, you may need to amend your return. The IRS may correct mathematical, clerical errors on a return and may accept returns without certain required forms or schedules. In these instances, there's no need to amend your return. However, do file an amended return if there's a change in your filing status, income, deductions, credits or tax liability.