Mont. Code Ann. § 28-2-803
28-2-803 Consideration to be lawful -- effect of illegality
28-2-803 . Consideration to be lawful -- effect of illegality. The consideration of a contract must be lawful within the meaning of 28-2-701 . If any part of a single consideration for one or more objects or of several considerations for a single object is unlawful, the entire contract is void.