In what circumstances can defendants bring an action for unjust enrichment?

In support of their appeal, defendants cite only Kesslar v. Sapp, 169 Cal.App.2d 818, 338 P.2d 34, to the effect that where a party stands upon a contract he may not rely upon an unjust enrichment theory. The cited authority presents the [244 Cal.App.2d 577] corollary of that proposition: that where money has been paid under a contract unenforceable because of uncertainty, an action to recover the money may be brought upon a theory of unjust enrichment.