In English law, a guarantee is a contract to answer for the payment of some debt, or the performance of some duty by a third person who is primarily liable for that payment or performance. It is a collateral contract, which does not extinguish the original obligation for payment or performance. It is rendered null and void if the original obligation fails. The liabilities of a guarantor in law depend upon those of the principal debtor, and when the principal's obligations cease the guarantor's do too,[3] except in certain cases where the discharge of the principal debtor is by the operation of the law.[4] If, for example, a person wrongly supposes that someone is liable to them, and a guarantee is given on that erroneous basis, the guarantee is invalid by virtue of the law of contracts, because its foundation (that another was liable) failed.[5]