With regard to the Form 1099-R provided by the payer, if it has code 3 (Disability) in box 7 and the distribution was received before the normal retirement date for the plan, the taxable amount of the distribution is reportable on 2017 line 7 as wages, not on line 16 as a pension distribution. If the taxable amount is truly zero as is indicated by $0 in box 2a and box 2b Taxable amount not determined being marked, this presents a situation where the IRS will not see that the distribution has been properly reported because it will appear nowhere on your filed tax return. This situation would require a separate explanation statement describing why the distribution is not taxable. (The IRS often fails to recognize these as being reportable as wages rather than as pension even when the amount is reported as fully taxable.)
If the distribution was received after the normal retirement date for the plan, it goes on line 16. The gross amount of the distribution would be included on line 16a but the $0 taxable amount would mean that none of it would appear on line 16b. However, in most cases where a distribution was received after the normal retirement date of the plan I would expect the Form 1099-R to have code 7 instead of code 3.
