A certificate of probable cause is required to challenge the validity of a prior conviction enhancement admitted by a defendant. A jury convicted appellant, and in a bifurcated proceeding, he agreed to admit a prison prior in exchange for dismissal of other enhancement allegations. Appellant then unsuccessfully tried to withdraw his admission. He appealed and sought a certificate of probable cause, which was denied. On appeal appellant claimed he did not knowingly and intelligently waive his rights when he admitted the prior. And he argued a certificate was not needed because Penal Code section 1237.5 is expressly limited to judgments of conviction upon a guilty or no contest plea. The court held the issue was not cognizable without a certificate. An admission of an enhancement which admits every element is a conclusive admission of guilt and is thus subject to the same principles as guilty pleas.