My all-time favorite sitcom joke

It's from AMOS & ANDY, a show from the '50s that is way too politically incorrect to ever air on commercial television again.   But this joke killed me when I first heard it, and makes me laugh every time.  And don't worry.  It's not racist.  Just cruel. 

I often talk about how important the set-up is in constructing jokes.  This is a perfect example.  It's the set up that makes this gag work.