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. 4.1 With reference to the applicable international legal instrument, summarise the circumstances which may deem a treaty invalid.