Minn. Stat. § 185.09 – Certain Acts Not Enforceable

(a) Any undertaking or promise such as is described in this section, or any other undertaking or promise in conflict with the public policy declared in section 185.08, is hereby declared to be contrary to the public policy of the state, shall not be enforceable in any court of the state, and shall not afford any basis for the granting of legal or equitable relief by any court, including specifically the following:
(b) Every undertaking or promise hereafter made, whether written or oral, express or implied, constituting or contained in any contract or agreement of hiring or employment between any individual firm, company, association, or corporation, and any employee or prospective employee of the same, whereby
1. (1) either party to such contract or agreement undertakes or promises not to join, become or remain a member of, any labor organization or of any employer organization; or
2. (2) either party to such contract or agreement undertakes or promises to withdraw from an employment relation in the event that the party joins, becomes or remains a member of, any labor organization or of any employer organization.