Betterment is the payer. According to their web site:
"We cannot make changes to Box 2a of the 1099-R. It is the customer’s responsibility to determine the appropriate taxable portion when filing their tax return by completing the Form 8606." That's why I was thinking I need an 8606. However.. reading 8606 instructions, it seems like I shouldn't even have to do this. I realized my error before the end of the tax year and corrected it AND I took a loss.
