155A.25  Burden of proof.
  In any complaint, information, or indictment, and in any action or proceeding brought for the enforcement of any provisions of this chapter, it shall not be necessary to negate any exception, excuse, proviso, or exemption contained in this chapter, and the burden of proof of any such exception, excuse, proviso, or exemption shall be upon the defendant.
87 Acts, ch 215, §25