It seems that your 401(k) plan made a mistake by not withholding the statutorily required 20% (unless the plan was an IRA rather than a 401(k)). Regardless, you'll want to make sure that your total withholding and estimated tax payments at satisfy one of the safe harbor requirements described above to avoid an underpayment penalty (or at least minimize the penalty if it's too late to make up for underpayment for the first 2 tax quarters of 2017). With little tax liability for 2016, it may not take much to meet the safe harbor of 100% (110%) of your 2016 tax liability.
