Cal. CIV § 1578

Mistake of law constitutes a mistake, within the meaning of this Article, only when it arises from: 1. A misapprehension of the law by all parties, all supposing that they knew and understood it, and all making substantially the same mistake as to the law; or, 2. A misapprehension of the law by one party, of which the others are aware at the time of contracting, but which they do not rectify.