The test for whether malperformance of a contract justifies cancellation by the innocent party, in the absence of an express lex commissoria, entails a value judgment by the Court. It is essentially a balancing of competing interests - that of the innocent party claiming rescission and that of the party who committed the breach. The ultimate criterion must be one of treating both parties, under the circumstances, fairly, bearing in mind that rescission, rather than specific performance or damages, is the more radical remedy. The question is whether the breach is so serious that it is fair to allow the innocent party to cancel the contract and undo all its consequences. The breach must go to the root of the contract, affect a vital part or term, relate to a material or essential term, or constitute a substantial failure to perform. More generally, the breach must be so serious that it would not be reasonable to expect that the creditor should retain the defective performance and be satisfied with damages to supplement the malperformance.