An offer of proof is generally required to preserve claims of error regarding the wrongful exclusion of evidence during a trial.
There is a limited exception to the offer of proof requirement. There are three requirements for this exception to apply: (1) the record must show a complete understanding of the excluded testimony, (2) the objection must be made to a category of evidence rather than to specific testimony, and (3) the record must show that the evidence would have helped its proponent. State Highway Comm’n v. Ne. Bldg. Co., 421 S.W.2d 297 (Mo. 1967).