Mont. Code Ann. § 31-2-342
31-2-342 Supplementary provisions
31-2-342 . Supplementary provisions. Unless displaced by the provisions of this part, the principles of law and equity, including the law merchant and the law relating to principal and agent, estoppel, laches, fraud, misrepresentation, duress, coercion, mistake, insolvency, or other validating or invalidating cause, supplement its provisions.