Mo. Rev. Stat. § 509.410: Objection that no demand has been made not available, when.

It shall not hereafter be available to a party as an objection that no demand for the subject matter of a suit was made prior to its institution, unless it is expressly set up by way of defense in the answer or replication, and is also accompanied with a tender of the amount that is due; in which case, if the plaintiff will further prosecute his suit, and shall not recover a greater sum than is tendered, he shall pay all costs. This provision shall be applicable as well to actions for property as for money; when property is tendered the damages for its detention, if any, shall also be tendered.