In that instance, it is actually the impossibility or impracticability defenses.
From the en.wikipedia.org
He gave them impracticability, the type of attack that every opposition team can't plan for.
From the couriermail.com.au
He talked about the impracticability of such an exercise, and implied that he lacked sufficient staffing.
From the guardian.co.uk
In the law of contracts, an act of God may be interpreted as an implied defence under the rule of impossibility or impracticability.
From the en.wikipedia.org
The hall would argue that the tornado was an act of God and excuses its nonperformance via impossibility or impracticability.
From the en.wikipedia.org
Vanbrugh's reputation still suffers from accusations of extravagance, impracticability and a bombastic imposition of his own will on his clients.
From the en.wikipedia.org
More examples
The quality of not being usable
The doctrine of impracticability in the common law of contracts excuses performance of a duty, where that duty has become unfeasibly difficult or expensive for the party who was to perform.
Substantial difficulty or inconvenience in following a particular course of action, but not such insurmountability or hopelessness as to make performance impossible.