Mo. Rev. Stat. § 143.997

Any provision of law to the contrary notwithstanding, where a statute or a rule promulgated by the director has excluded any income from the computation of tax imposed under this chapter and is thereafter held by a final decision of the Missouri supreme court to be invalid, the director shall not be entitled to serve a notice of deficiency of taxes which result from such court decision for any period occurring prior to the date of the court's mandate or the implementation of regulations interpreting such court decision, whichever is later.