Under the circumstances of this case, the Compassionate Use Act (Health and Saf. Code, sec. 11362.5) did not provide defendant with a defense. Defendant was found in possession of about 4.74 ounces of marijuana in his car. At trial defendant presented a certificate from his doctor, indicating treatment for traumatic arthritis and major depression, and bearing this limitation: “no more than 10 plants.” Defendant presented no other evidence. He did not present evidence as to his medical condition, his need for marijuana, how much he used at a time, or how often he used it. He produced no evidence of how much marijuana 10 plants could produce. The court rejected his argument that his belief that the Act provided him with a defense was a mistake of fact. Instead, the court found it was a mistake of law, which did not criminally excuse his conduct.