Wis. Stat. § 452.20: Limitation on actions for commissions.

No person engaged in the business or acting in the capacity of a broker or salesperson within this state may bring or maintain an action in the courts of this state for the collection of a commission or compensation for the performance of any act mentioned in this chapter without alleging and proving that he or she was a duly licensed broker or salesperson at the time the alleged cause of action arose.