Mont. Code Ann. § 32-1-447

It is unlawful for any bank to pledge, mortgage, or hypothecate to any depositor any of its real or personal property as security for any deposit, and any pledge, mortgage, or hypothecation made in violation thereof is unenforceable. This provision does not apply to any deposits of money of the United States, public funds deposited in accordance with the provisions of any depository act of this state or the United States, or bankruptcy estate funds or deposits, including deposits of receivers or trustees in bankruptcy, deposited under the direction and supervision of a court of record of the state of Montana or of the United States.