This means that the plan custodian did not have the information needed to determine the taxable amount. Leave box 2a blank, do not enter "0".

If box 2a is blank (there's nothing in there), that doesn't means the box 1 amount is nontaxable. Rather, it is up to you to determine from your records the nontaxable amount to enter in box 2a.

Thanks.

Maybe,  If you see a 0 (zero) in Box 2a of your 1099-R paper form, the entire distribution amount in Box 1 is nontaxable.  However, if Box 2a is blank (empty) on your 1099-R form, the taxable portion of your distribution needs to be calculated.  

Is my distribution nontaxable if Box 2a on my 1099... (intuit.com)