California Code, Civil Code - CIV § 683.1

No contract or other arrangement made after the effective date of this section between any person, firm, or corporation engaged in the business of renting safe-deposit boxes and the renter or renters of a safe-deposit box, shall create a joint tenancy in or otherwise establish ownership in any of the contents of such safe-deposit box.  Any such contract or other arrangement purporting so to do shall be to such extent void and of no effect.