Hmm. I believe you are right, but I don't want to rule anything out just yet.

[A pause, and then their words are deliberate. The truth is that they haven't considered it beyond some passing thought.] I haven't. I don't have the faintest idea what it could be, either. It's clear to me that I must have one, but I wouldn't know how to begin seeking it out.

What did you do?