17. Common mistake as to existence of subject matter or its essential element.

Where parties enter into a contract under a common mistake as to the existence of the subject matter or of some fact or facts forming an integral element of the subject matter, it is a question of construction as to whether either or both of them is or are relieved of liability to perform1. In most such cases, both parties are relieved of liability, because the consideration for which each party contracted has failed2 and, deprived of any effective content, the contract has the appearance of having been void ab initio3.

Where the contract concerned is one for the sale