California Code, Business and Professions Code - BPC § 143

(a) No person engaged in any business or profession for which a license is required under this code governing the department or any board, bureau, commission, committee, or program within the department, may bring or maintain any action, or recover in law or equity in any action, in any court of this state for the collection of compensation for the performance of any act or contract for which a license is required without alleging and proving that he or she was duly licensed at all times during the performance of that act or contract, regardless of the merits of the cause of action brought by the person.

(b) The judicial doctrine of substantial compliance shall not apply to this section.

(c) This section shall not apply to an act or contract that is considered to qualify as lawful practice of a licensed occupation or profession pursuant to Section 121.