Mo. Rev. Stat. § 521.620

If the petition denying the cause of action be not verified by oath, or if the petition allege a setoff or other collateral avoidance of the original cause of action, the petitioner shall be required to prove his allegations, and, on his failure to do so, his petition shall be dismissed, and the original judgment shall stand absolute; and if any part thereof remain unpaid, a general judgment shall be rendered against him for the balance remaining unpaid.