Mo. R. Civ. P. 55.08
In pleading to a preceding pleading, a party shall set forth all applicable affirmative defenses and avoidances, including but not limited to accord and satisfaction, arbitration and award, assumption of risk, contributory negligence, comparative fault, state of the art as provided by statute, seller in the stream of commerce as provided by statute, discharge in bankruptcy, duress, estoppel, failure of consideration, fraud, illegality, injury by fellow servant, laches, license, payment, release, res judicata, statute of frauds, statute of limitations, truth in defamation, waiver, and any other matter constituting an avoidance or affirmative defense. A pleading that sets forth an affirmative defense or avoidance shall contain a short and plain statement of the facts showing that the pleader is entitled to the defense or avoidance. When a party has mistakenly designated a defense as a counterclaim or a counterclaim as a defense, the court may treat the pleadings as if there had been a proper designation.
Mo. R. Civ. P. 55.08