The prosecution is not required to prove, as an element of the “truth of a prior conviction” the validity of the underlying plea. Therefore, where the trial court found one of appellant’s prior “strikes” not to be true because the prosecution failed to meet its burden of providing the validity of the Boykin-Tahl waiver, the sentence had to be vacated and the case remanded with directions to conduct a new trial on the truth of the prior conviction. The trial court erred when it invalidated the prior conviction on constitutional grounds without complying with the procedures established in People v. Sumstine (1984) 36 Cal.3d 909, requiring the defendant to challenge the constitutionality of a prior plea. The prosecution was never put on notice of any “challenge” to the prior conviction. The prosecution could properly appeal from the court’s invalidation of the prior “strike.” The court did not make a “not true” finding, but invalidated the “strike” on constitutional grounds. Accordingly, the order was appealable under Penal Code section 1238, subdivisions (a)(1) and (a)(10).