Cal. Rev. & Tax. Code § 7659.6

Any person required to make a prepayment pursuant to Section 7659.1 who fails to make a prepayment before the last day of the month following the monthly period for which the prepayment became due and who files a timely return and payment for the monthly period for which the prepayment became due shall pay a penalty of 6 percent of the amount equal to 95 percent of the tax liability for the monthly period for which the required prepayment was not made.