When the performance of a contract requires the existence of a specific thing, and at the time the contract is made the thing does not exist, it is obvious that completion of the transaction is impossible, and that the parties would not have entered into the agreement had they known the facts. Though the agreement is impossible of performance it does not, however, necessarily follow that the party who has in terms undertaken the performance which requires the existence of the thing in question, is not liable for failing to perform. It is necessary to distinguish cases where he takes the risk of his ability from cases where both parties assume the existence of the thing. In the latter case it is inequitable to charge the promisor. The situation is possible both where real estate and where personal property is the non-existing object.