Русско-англо-русский юридический словарь

The narrower form of the exception is based on the idea that state A may not complain of the non-performance of an obligation owed to it by state B where such performance has been prevented by the prior wrongful act of state A. (Australian Year Book of International Law)

A non-performance of an obligation is fundamental if strict compliance with the obligation which has not been performed is of essence to the contract; ... (Unidroit)