Read this: A state's consent may be invalidated if there was an erroneous understanding of a fact or situation at the time of conclusion, which formed the "essential basis" of the state's consent. Consent will not be invalidated if the misunderstanding was due to the state's own conduct, or if the truth should have been evident.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): What, if it formed the "essential basis" of a state's consent to a treaty, may invalidate that consent?
an erroneous understanding of a fact or situation