Mont. Code Ann. § 16-6-204

The description of any offense under this code, in the words of this code or in any words of like effect, shall be sufficient in law; and any exception, exemption, provision, excuse, or qualification, whether it occurs by way of proviso or in the description of the offense in this code, may be proved by the defendant but need not be specified or negatived in the information or complaint; but if it is so specified or negatived, no proof in relation to the matter so specified or negatived shall be required on the part of the informant or complainant.