Depending on a standard of duty which a party has agreed to assume under the contract he may find himself liable for breach even "without what one might call fault on their part".
And this difference was taken, that where the law creates a duty or charge, and the party is disabled to perform it without any default in him, and hath no remedy over, there the law will excuse him. As in the case of waste, if a house be destroyed by tempest, or by enemies, the lessee is excusedbut when the party by his own contract creates a duty or charge upon himself, he is bound to make it good, if he may, notwithstanding any accident by inevitable necessity, because he might have provided against it by his contract. And therefore if the lessee covenant to repair a house, though it be burnt by lightning, or thrown down by enemies, yet he ought to repair it. Dyer 33. a. 40 E. 3. 6. h. Now the rent is a duty created by the parties upon the reservation, and had there been a covenant to pay it, there had been no question but the lessee must have made it good, notwithstanding the interruption by enemies, for the law would not protect him beyond his own agreement
Per Lord Wright in in Joseph Constantine Steamship Line Ltd v Imperial Smelting Corporation Ltd  AC 154 at p. 184:
a dictum unnecessary to the decision of the Court of King’s Bench in Paradine v Jane (1647) Aleyn, 26 is often quoted: "When the party, by his own conduct, creates a duty or charge upon himself, he is bound to make it good, if he may, notwithstanding any accident by inevitable necessity, because he might have provided against it by his contract." It is true that a contract, absolute in terms, may be absolute also in effect. The contractor, if he cannot perform, must pay damages. Prima facie, the actual language governs. But a contract absolute in terms, is not necessarily absolute in effect. It is in all cases a question of construction, as Lord Cranworth L.C. pointed out in Couturier v Hastie (1856) 5 H. L. Cas. 673, 681, a case where under a contract for the sale of goods, the goods had perished at the date of the contract, both parties being then ignorant of the fact. Lord Cranworth said that, looking at the contract alone, what the parties contemplated, those who bought and those who sold, was that there was an existing something to be bought and sold at the time of the contract. The buyer could not be held liable for the price, or the seller for failure to deliver. The court having thus construed the contract, both parties were held in the event to be discharged in law.