if you use the IRS tool (see link) it will say you need to report it. I would file an amended return. while you know that it's not taxable the IRS doesn't. also, the IRS want's form 8606 - this is where you'll show it's not taxable.
https://apps.irs.gov/app/IPAR/investigate/IPAR_1/en-US/Attribute~retro_interview_complete~global~glo...
