Correct in general. However, if you filed late in the season, the AGI may not be in the database by the time you try to file your 2018 return in 2019 (yes, the IRS is slow, big surprise). If you are rejected, you can try zero, and if that doesn't work you will have to mail your return next year too. But if you file on time next year, they should accept your AGI in 2020.
