I don't know if this is still a bug in Emacs 21; I just checked the code
quickly (didn't test), but it was a bug in Emacs 20:

Initially, the kill-ring is nil, and mouse-save-then-kill expects it to be a
consp. I fixed this for myself in Emacs 20 this way: (unless kill-ring
(kill-new ""))

 - Drew