I rolled over $17k of after tax 401k to traditional IRA in 2016 and then converted it to Roth IRA. Where do I enter that the $17k is after tax? 1099-R 2b is checked.

Fidelity 401k 1099-R shows the withdrawal amount in box 2a as $0 taxable.  The Fidelity check went directly to traditional IRA at Vanguard.  I then converted the traditional IRA to Roth.  Vanguard 1099-R shows the full amount as taxable in box 2a with box 2b checked for taxable amount not determined.  I can't figure out where to enter that the amount shouldn't be taxed.  Thanks, Scott