Mo. Rev. Stat. § 400.2A-109: Option to accelerate at will.

A term providing that one party or his successor in interest may accelerate payment or performance or require collateral or additional collateral "at will" or "when he deems himself insecure" or in words of similar import must be construed to mean that he has power to do so only if he in good faith believes that the prospect of payment or performance is impaired.