Mo. Rev. Stat. § 491.350: Penalty for failure to obey subpoena without excuse.

If any witness so attached shall show a reasonable excuse for not appearing, as required by the subpoena, he shall be discharged, either with or without payment of the costs of the attachment, in the discretion of the associate circuit judge; but if he fail to show any good and sufficient reason for not attending, he shall be fined in such sum as the associate circuit judge shall think reasonable to impose, not exceeding twenty dollars, and may be committed until such fine and costs are paid, in like manner as persons may be committed for the nonpayment of fines and costs in criminal cases.