Is a defendant entitled to a certificate of probable cause to challenge his sentence?

Finally, defendant contends that his sentence was illegal because the trial court failed to honor a promise in the plea bargain to strike prior convictions for purposes of sentencing; and that appellate counsel rendered ineffective assistance by not raising this issue on appeal. A certificate of probable cause is unnecessary to challenge matters that occur after entry of the plea and do not go to the validity of the plea. (People v. Buttram (2003) 30 Cal.4th 773, 780; 1237.5; Cal. Rules of Court, rule 8.304(b).) However, defendant's sentencing contentions have no merit.