Penalty for estimated payments underpayment. I understand that if the total tax due is under $1,000, (it is,) there is no penalty. Turbo Tax says I own $9 in penalty.

tax due for the year is under $800. Estimated payments were just a bit shy of the voucher amounts and a bit irregular, but total tax due is under $1,000. Why is the software generating a $9 small penalty?