The rule is that when (1) one party enters a contract under a mistaken assumption that is (2) basic to the contract, (3) the contract is voidable by the party adversely affected by the mistake, if: (4) that party ought not to bear risk of the mistake (as determined by the criteria summarized in Restatement (Second) Contracts ยง 154), and: (5) it would be "unconscionable" to enforce the contract; or, the other party had reason to know of the mistake, or the mistake was the other party's fault.