Legal Principle: Where both parties to an agreement are under a mistake as to matter of fact essential to the agreement, the agreement is void. Factual Situation: A, contracts with B to buy a necklace, believing it to be made of very costly rare pearls, while in reality, the pearls are only an imitation having very little value. B knows that A is mistaken but does not disclose the facts. Issue: Is A bound by the contract? Decision: