In People v. Panizzon (1996) 13 Cal. 4th 68, 79 [51 Cal. Rptr. 2d 851, 913 P.2d 1061] (Panizzon), the California Supreme Court clarified that "a challenge to a negotiated sentence imposed as part of a plea bargain is properly viewed as a challenge to the validity of the plea itself" and therefore requires that the defendant "seek and obtain a probable cause certificate in order to attack the sentence on appeal." In Panizzon at page 79, the state Supreme Court concluded that "a challenge to a negotiated sentence imposed as part of a plea bargain is properly viewed as a challenge to the validity of the plea itself" and thus subject to compliance with section 1237.5. There, the defendant pleaded no contest to various felony counts, admitted a weapons use enhancement, and agreed to a sentence of life with the possibility of parole plus 12 years, in exchange for the dismissal of additional counts. After the defendant was sentenced in conformity with the plea bargain, he filed a notice of appeal, claiming that his sentence was disproportionate to the sentences imposed upon his codefendants and thus violative of the federal and state constitutional prohibitions against cruel and unusual punishment. In rejecting the defendant's contention that his appeal was based on a postplea error (for which a probable cause certificate was not required) because he was claiming that the sentence was unconstitutional in comparison to the sentences imposed on his codefendants based on events that occurred after his no contest plea, the high court observed that "all the trial court did here was to sentence defendant in accordance with the previously entered plea." (13 Cal. 4th at p. 78) It ruled: "That the events supposedly giving rise to defendant's disproportionality claim occurred afterwards . . . is of no consequence. Rather, 'the crucial issue is what the defendant is challenging.' The high court explained the rationale underlying its ruling in Panizzon as follows: "In determining whether section 1237.5 applies to a challenge of a sentence imposed after a plea of guilty or no contest, courts must look to the substance of the appeal: 'the crucial issue is what the defendant is challenging, not the time or manner in which the challenge is made.' Hence, the critical inquiry is whether a challenge to the sentence is in substance a challenge to the validity of the plea, thus rendering the appeal subject to the requirements of section 1237.5." (13 Cal. 4th at p. 76, italics in original.)