What is the burden of proving a contract unenforceable?

⁸ Walton v. City of Red Bluff, supra, 2 Cal.App.4th at page 131, 3 Cal.Rptr.2d 275, states: "[w]here a claim is unenforceable the burden is on the defense to demonstrate unenforceability. When a claim is barred by the statute of limitations the issue must be pleaded as an affirmative defense. [Citation.] Other claims of unenforceability must be raised by affirmative defense, such as a claim a contract is barred by the statute of frauds [citations] [waiver at trial by failure to object], res judicata [citations] [same], release [citation], or accord and satisfaction [citation]." (Italics added.)

^** See footnote *, ante.