Oh okay hmm - then I'd guess it might just be a misplaced/missing bracket somewhere. Just make sure that you set an option for ALL the possible dialogue choices. If there's two dialogue choices, there should be two values it's going to change to.
myChangeVars[dialogue_line_number] = [obj_id, "choice_variable_as_string", ["value1", "value2"]];
If you're still having trouble could you send me a screenshot of the code where you declare this?