Can a defendant bring a motion for a certificate of probable cause?

We requested supplemental briefing from the parties as to whether defendant's contention amounted in substance to an attack on the validity of the plea, which may not be brought without a certificate of probable cause. ( 1237.5, subd. (b); People v. Panizzon (1996) 13 Cal.4th 68, 76.) Having considered the parties' original and supplemental briefs, we conclude that defendant, in fact, seeks to attack the validity of his plea. Since he did not obtain a certificate of probable cause, we dismiss the appeal.