Sec. 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:
(1)  an official statement of the law contained in a written order or grant of permission by an administrative agency charged by law with responsibility for interpreting the law in question;  or
(2)  a written interpretation of the law contained in an opinion of a court of record or made by a public official charged by law with responsibility for interpreting the law in question.
(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.
Acts 1973, 63rd Leg., p. 883, ch. 399, Sec. 1, eff. Jan. 1, 1974.  Amended by Acts 1993, 73rd Leg., ch. 900, Sec. 1.01, eff. Sept. 1, 1994.