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, and: (5) it would be "unconscionable" to enforce the contract; or, (6) the other party had reason to know of the mistake, or the mistake was the other party's fault.