In $2005,$ State C and State D concluded a border demarcation agreement. In $2006,$ the parliaments of State C and State D ratified the agreement. The agreement was subsequently deposited with the United Nations Secretary$-$General. The Constitution of State D includes a provision stating that laws altering the borders of the State can only be adopted with a $2/3$ majority of the parliament $-$ the border demarcation agreement was approved with only a simple majority. In $2023,$ a Court in State C found that the ratification of the agreement was invalid. with reference to the applicable international legal instrument, summarise the circumstances which may deem a treaty invalid.