Restatement Second of Contracts § 198
Restitution in Favor of Party Who Is Excusably Ignorant or Is Not Equally in the Wrong
A party has a claim in restitution for performance that he has rendered under or in return for a promise that is unenforceable on grounds of public policy if
- he was excusably ignorant of the facts or of legislation of a minor character, in the absence of which the promise would be enforceable, or
- he was not equally in the wrong with the promisor.
Copyright, The American Law Institute