Texas Penal Code - PENAL § 8.03. Mistake of Law

(a) It is no defense to prosecution that the actor was ignorant of the provisions of any law after the law has taken effect.

(b) It is an affirmative defense to prosecution that the actor reasonably believed the conduct charged did not constitute a crime and that he acted in reasonable reliance upon:

(c) Although an actor's mistake of law may constitute a defense to the offense charged, he may nevertheless be convicted of a lesser included offense of which he would be guilty if the law were as he believed.