What is the test for the defence in a personal injury case?

There is, however, another principle upon which some of these claims can be supported as a defence if we eliminate the defendants’ demand for damages, and that is, the principle laid down in Williams v. Price, 1 Sim. & S. 581 (57 E.R. 229), the headnote of which reads as follows: Where a creditor takes from his debtor an assignment of a debt due from a third person as a security for his demand, and, by his wilful default, the debt becomes irrecoverable, he must bear the loss.