Mo. Rev. Stat. § 546.020: Plea of not guilty, not formally tendered.

When a person shall be arraigned upon any indictment or information, it shall not be necessary to ask him how he will be tried; and if he deny the charge in any form, or require a trial, or if he refuse to plead or answer and in all cases when he does not confess the charge to be true, a plea of not guilty shall be entered, and the same proceedings shall be had, in all respects, as if he had formally pleaded not guilty to such indictment or information; provided, however, that no judgment rendered in any criminal case shall be reversed, set aside or for naught held for the reason that the record does not show that the defendant was arraigned and a plea of not guilty entered, where a trial was had in all respects as though the defendant had been arraigned and had formally tendered the general issue under a plea of not guilty.