A guarantor is not liable if the contract of the principal is unlawful, but the guarantor is liable notwithstanding any mere personal disability of the principal, even though the disability would make the contract void against the principal.
Mont. Code Ann. § 28-11-203
Liability under guaranty of illegal contract or contract void against principal
En
Official source: Montana Code Annotated (Montana Legislature). Reproduced from public-domain Montana statutes; confirm against the official source for the current text. Not legal advice.