Article 47 Vienna Convention on the Law of Treaties

Specific restrictions on authority to express the consent of a State

If the authority of a representative to express the consent of a State to be bound by a particular treaty has been made subject to a specific restriction, his omission to observe that restriction may not be invoked as invalidating the consent expressed by him unless the restriction was notified to the other negotiating States prior to his expressing such consent.

See also: Article 46 Vienna Convention on the Law of Treaties