Thanks, setting it to 0 worked. In 2019, the IRS received my mailing (2018 return + check), cashed the check, and lost the return. They emailed in late December that I hadn't filed, so I sent in a duplicate return by mail then.
The AGI used to accept e-filed returns must be a copy of the original value, defaulted to 0 before the end of the year. Apparently the IRS agents don't know that.
Thanks for your help!
PS. I found I had to change the value to 0 twice: the first time, I changed it to 0 and hit the continue button, and turbo-tax changed it back to the 2018 AGI. I changed it again, and then the 0-value "stuck".
