Montana Title 45. Crimes § 45-5-611. Bigamy

(a) the offender believes on reasonable grounds that the prior spouse is dead;

(b) the offender and the prior spouse have been living apart for 5 consecutive years throughout which the prior spouse was not known by the offender to be alive;

(c) a court has entered a judgment purporting to terminate or annul any prior disqualifying marriage and the offender does not know that judgment to be invalid; or

(d) the offender reasonably believes that the offender is legally eligible to remarry.