Mo. Rev. Stat. § 379.155
No fire insurance policy which may be issued after this section takes effect shall contain any clause or provision requiring the assured to take out or maintain a larger amount of insurance than that covered by such policy, nor in any way providing that the assured shall be liable as coinsurer with the company issuing the policy for any part of the loss or damage which may be occasioned by fire or lightning to the property covered by such policy, nor making provisions for a reduction of such loss or damage, or any part thereof, by reason of the failure of the assured to take out and maintain other insurance upon said property. And all clauses and provisions in fire policies, issued after the taking effect of this section, in contravention of the prohibitions in this section contained, shall be ab initio void and of no effect; provided, that the provisions of this section shall not apply to policies issued upon personal property in cities which now contain or which may hereafter contain one hundred thousand inhabitants or more whenever the insured sign an agreement endorsed across the face of said policy to be exempt from the provisions thereof.
§ 379.155, RSMo