Section 921

Every promise made after August 21, 1933, between any employee or prospective employee and his employer, prospective employer or any other person is contrary to public policy if either party thereto promises any of the following:

(a) To join or to remain a member of a labor organization or to join or remain a member of an employer organization,

(b) Not to join or not to remain a member of a labor organization or of an employer organization,

(c) To withdraw from an employment relation in the event that he joins or remains a member of a labor organization or of an employer organization.

Such promise shall not afford any basis for the granting of legal or equitable relief by any court against a party to such promise, or against any other persons who advise, urge, or induce, without fraud or violence or threat thereof, either party thereto to act in disregard of such promise.