Mistake of law constitutes a mistake, within the meaning of this part, only when it arises from: (1) a misapprehension of the law by all parties, all supposing that they knew and understood it and all making substantially the same mistake as to the law; or (2) a misapprehension of the law by one party of which the others are aware at the time of contracting but which they do not rectify.
What Constitutes Mistake Of Law
Mont. Code Ann. § 28-2-410, under Circumstances Which Affect Validity of Apparent Consent.
Mont. Code Ann. § 28-2-410