California Code, Code of Civil Procedure - CCP § 412.30

In an action against a corporation or an unincorporated association (including a partnership), the copy of the summons that is served shall contain a notice stating in substance: “To the person served: You are hereby served in the within action (or special proceeding) on behalf of (here state the name of the corporation or the unincorporated association) as a person upon whom a copy of the summons and of the complaint may be delivered to effect service on said party under the provisions of (here state appropriate provisions of Chapter 4 (commencing with Section 413.10) of the Code of Civil Procedure).” If service is also made on such person as an individual, the notice shall also indicate that service is being made on such person as an individual as well as on behalf of the corporation or the unincorporated association.

If such notice does not appear on the copy of the summons served, no default may be taken against such corporation or unincorporated association or against such person individually, as the case may be.