Mo. Rev. Stat. § 143.681

In the event that the amount of tax is understated on the taxpayer's return due to a mathematical error, the director of revenue shall notify the taxpayer that an amount of tax in excess of that shown on the return is due and has been assessed. Such a notice of additional tax due shall not be considered a notice of deficiency nor shall the taxpayer have any right to file a protest under section 143.631.