Where defendant was denied a full and fair hearing on his challenge to the validity of a strike prior, he may challenge an enhanced sentence for a later offense on the ground the prior conviction was unconstitutionally obtained. When he pled guilty to criminal threats (Pen. Code, § 422) in 2000, Dubrin was assured by the trial court and prosecutor that the offense would not count as a strike. However, Proposition 21, which was approved by voters the day before Dubrin’s plea, added a section 422 offense as a strike and there were no issues of retroactivity. (People v. Ringo (2005) 134 Cal.App.4th 870, 884.) After he learned the section 422 conviction was a strike, Dubrin filed petitions in the state courts challenging the conviction. The Court of Appeal denied relief, finding Dubrin was not “in custody.” In 2008, Dubrin was convicted of new felonies and given a life sentence based on two strike priors. His renewed challenges to the section 422 prior were denied and he sought federal writ relief. Held: Reversed. Generally when an “expired” prior conviction is used to enhance a later sentence, the defendant may not challenge the prior via a federal writ petition on the ground it was unconstitutionally obtained. The rule is based on the need for finality of convictions and ease of administration. (Lackawanna County Dist. Atty. v. Coss (2001) 532 U.S. 394.) However, an exception to the rule arises when a defendant, exercising due diligence, does not receive a full and fair hearing on his timely challenge to a prior. Here, the plea court was incorrect in telling Dubrin his offense would not be a strike. The Court of Appeal erred when it summarily denied relief on the basis that Dubrin was not in custody, as he was on parole. The state courts, without justification, refused to rule on Dubrin’s constitutional claim on postconviction review. As Dubrin cannot be faulted for failing to obtain timely review of his constitutional challenge to an expired prior conviction, and that conviction was used to enhance his current sentence, he may challenge the prior as unconstitutional in federal habeas proceedings.