Oddly, I can't reproduce the problem, either.
However, I suspect the error is legitimate. Prior to evaluation, it's not a valid JSON document due to the {{}} notation. That could only be a JSON string which would need to be quoted.
Of course, when the {{}} is evaluated, it becomes an integer which isn't supposed to be evaluated.
So, it evaluates to a valid JSON document - but it's not valid in the editor prior to the evaluation.
This is probably just the intended behavior of the editor.
--Mike