I understand the concern: an oathbreaker taking an oath not to lie about the substance of the broken oath.
It's not just that. There's a whole mini-industry that has grown up to assist people who "need" annulments - they can tell you what to say, how to say it, etc.
It's my conviction that there are very few married people, never mind married Catholics, who did not INTEND marriage on their wedding day. And yet, defective intent is annulling 60 000 marriages a year?
I think the bishops who preside over this would just as soon be done with it.