Wis. Stat. § 452.20
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.
Wis. Stat. § 452.20
A foreign corporation that contracts to sell a radio station without being licensed in this state cannot sue for its commission. The licensing requirement is not an unlawful burden on interstate commerce. Chapman Co. v. Service Broadcasting Corp. 52 Wis. 2d 32, 187 N.W.2d 794. The prohibition against paying a commission to an unlicensed person acting as a broker is absolute and not conditioned on the innocence or lack of sophistication of the other contracting parties. Because a contract for a commission with a person not licensed as a broker is void from its inception, estoppel is not available to avoid the effect of the statute. Greenlee v. Rainbow Auction/Realty Co. 202 Wis. 2d 653, 553 N.W.2d 257 (Ct. App. 1996), 95-1463. This section not applicable since broker as owners' agent was not attempting to negotiate the lease when conferring with his alleged principal in Wisconsin and therefore was not acting in the capacity of a broker in Wisconsin. Negotiation of lease occurred when broker met and conferred with prospective lessee either in Illinois or Tennessee. Paulson v. Shapiro, 490 F.2d 1.