Appellant was convicted of receiving a stolen vehicle, in violation of Penal Code section 496d. On appeal, he contended there was insufficient evidence to support his conviction because his honest if mistaken belief that the motorcycle was abandoned negated the felonious intent element of the offense. He also contended that the trial court prejudicially erred when it failed to instruct the jury on the mistake of fact and claim of right defenses, which would have negated the intent element of the offense. The appellate court found that the trial court erred when it failed to instruct the jury on both defenses. Appellant presented substantial evidence supporting his claim that the vehicle was abandoned. It was an old, rusty motorcycle found near trash bins, and had cobwebs on the wheels. Further, when he was stopped riding the motorcycle, he was informed that it was not reported stolen. After he determined who the registered owner was, he went to him to try to persuade him to sign over the vehicle to him. The evidence showed he had a good faith belief that the motorcycle had been abandoned. If substantial evidence supports the defense, a trial court must sua sponte instruct the jury. Applying the Watson standard here, the error in failing to instruct was prejudicial. It was reasonably probable that appellant would have obtained a more favorable result if the jury had been properly instructed on his defense, and therefore reversal was required.