stumpwm, ediff in emacs

I am posting my problem here in the hope that someone may have
experienced something similar.

In short, stumpwm hangs whenever I use ediff in emacs. I get this error
message, which I'm afraid means nothing to me:

--8<---------------cut here---------------start------------->8---
Caught! The value -s is not of type (UNSIGNED-BYTE 29) when binding
XLIB::VALUE
--8<---------------cut here---------------end--------------->8---

The system hangs no matter the value of the variable
"ediff-window-setup-function". I have no problems using ediff with other
desktop managers. My stumpwm version is (SBCL 2.1.3).

Thanks.

Colin Baxter.