Section 1237.5 states, "No appeal shall be taken by the defendant from a judgment of conviction upon a plea of guilty or nolo contendere, or a revocation of probation following an admission of violation, except where both of the following are met: (a) the defendant has filed with the trial court a written statement, executed under oath or penalty of perjury showing reasonable constitutional, jurisdictional, or other grounds going to the legality of the proceedings. (b) the trial court has executed and filed a certificate of probable cause for such appeal with the clerk of the court." Section 1237.5 is not a pointless technicality. by requiring a defendant who has pleaded guilty to obtain a certificate of probable cause to appeal, section 1237.5 promotes judicial economy by weeding out frivolous guilty plea appeals. (People v. Mendez (1999) 19 Cal.4th 1084, 1095.) "Its assumption is that, as a general matter, a judgment of conviction entered on a defendant's plea of guilty or nolo contendere does not present any issue warranting relief on appeal, and hence should not be reviewed thereon." (Id. at p. 1097.) In People v. Ribero (1971) 4 Cal.3d 55, the Supreme Court looked to the substance of the alleged error in deciding whether the defendant could forego the requirements of section 1237.5 in challenging the trial court's refusal to let him withdraw his guilty plea. Finding defense counsel's alleged misrepresentations in the plea process went to the validity of the plea itself, the Supreme Court ruled the defendant could not challenge the trial court's decision without complying with section 1237.5. (Id. at pp. 62-63.)