Vienna_Treaties1_1_1969 (pdf)
https://legal.un.org/avl/ha/vclt/vclt.html (UN URL)
Article 26
“Pacta sunt servanda”
Every treaty in force is binding upon the parties to it and must be performed by them in good faith.
Article 27
Internal law and observance of treaties
A party may not invoke the provisions of its internal law as justification for its failure to perform a treaty. This rule is without prejudice to article 46.
Article 46
Provisions of internal law regarding competence to conclude treaties
1. A State may not invoke the fact that its consent to be bound by a treaty has been expressed in violation of a provision of its internal law regarding competence to conclude treaties as invalidating its consent unless that violation was manifest and concerned a rule of its internal law of fundamental importance.
2. A violation is manifest if it would be objectively evident to any State conducting itself in the matter in accordance with normal practice and in good faith.