I made California estimated tax payments equaling 100% of my prior year's total tax liability, and yet this year, TTAX says I owe underpayment penalty. Why? How to fix?

My understanding is by making CA estimated tax payments equaling 100% of my prior year's total tax liability, I will be in "safe harbor" and will NOT be charged underpayment penalty even if I owe CA taxes for this year's return.