Civil Code § 1577

Mistake of fact is a mistake, not caused by the neglect of a legal duty on the part of the person making the mistake, and consisting in:

1. An unconscious ignorance or forgetfulness of a fact past or present, material to the contract; or,

2. Belief in the present existence of a thing material to the contract, which does not exist, or in the past existence of such a thing, which has not existed.