Mo. Rev. Stat. § 521.640
Current with changes from the 2023 Legislative Session
When the petition alleges a setoff, or other collateral avoidance of the cause of action, the plaintiff may answer the same, as in ordinary actions; and, in default of such answer, judgment may be taken in like manner and with like effect as in ordinary actions.
§ 521.640, RSMo
RSMo 1939 § 1502