Yes, the conversion must take place in the same year as the nondeductible IRA. You need to complete Form 8606 and submit , along with your 1099-R and a copy of Form 5498. You will receive a letter from the IRS if you don't. I started doing the conversion in 2015 and I have managed to receive 2 letters from the IRS and the documentation mentioned above fixed it both times.
I have yet to get the whole thing to work in Turbo-Tax, there are no clear instructions. It increases my taxable amount and there is no Form 5498 available.
https://www.irs.gov/publications/p590a
https://www.irs.gov/forms-pubs/about-form-8606
