Double whammy question. Not only do you have to talk about a weakness, but you have to second guess your bosses views.

You could sidestep this question and suggest that: "If you're serious about offering me this position, then I suggest you contact my boss to ask him/her."

I prefer taking this bull by the horns: "Not being a mind-reader, I can tell you what he/she has said to me." Then refer back to point 3 above. You'll seldom have to answer this question and point 3 in the same interview.

I like following up with: "May I ask if any kinds of employee traits have or are providing a challenge to performance?"

Turn the spotlight back on your interviewer to find out if there was something behind their question. Better you find out now and not after you've joined...