Section 15.20. Effect of ignorance or mistake upon liability

1.  A person is not relieved of criminal liability for conduct because
  he engages in such conduct under a mistaken belief of fact, unless:
    (a) Such factual mistake negatives the culpable mental state  required
  for the commission of an offense; or
    (b)  The  statute  defining  the  offense or a statute related thereto
  expressly provides that such factual mistake constitutes  a  defense  or
  exemption; or
    (c)  Such  factual  mistake  is  of  a kind that supports a defense of
  justification as defined in article thirty-five of this chapter.
    2. A person is not relieved of criminal liability for conduct  because
  he  engages in such conduct under a mistaken belief that it does not, as
  a matter of law, constitute an offense, unless such mistaken  belief  is
  founded upon an official statement of the law contained in (a) a statute
  or  other  enactment,  or  (b)  an  administrative  order  or  grant  of
  permission, or (c) a judicial decision of a state or federal  court,  or
  (d)  an  interpretation  of  the statute or law relating to the offense,
  officially made or issued by a public servant, agency  or  body  legally
  charged   or   empowered   with   the  responsibility  or  privilege  of
  administering, enforcing or interpreting such statute or law.
    3. Notwithstanding the use of the term "knowingly" in any provision of
  this chapter defining an offense in which the  age  of  a  child  is  an
  element  thereof, knowledge by the defendant of the age of such child is
  not an element of any such offense and it is not,  unless  expressly  so
  provided, a defense to a prosecution therefor that the defendant did not
  know  the  age  of  the  child or believed such age to be the same as or
  greater than that specified in the statute.
    4. Notwithstanding the use of the term "knowingly" in any provision of
  this chapter defining an offense in which  the  aggregate  weight  of  a
  controlled  substance  or  marihuana  is  an  element,  knowledge by the
  defendant of the  aggregate  weight  of  such  controlled  substance  or
  marihuana  is  not  an element of any such offense and it is not, unless
  expressly so provided, a defense to  a  prosecution  therefor  that  the
  defendant  did not know the aggregate weight of the controlled substance
  or marihuana.