[Sending this to mdiers & xemacs-beta as advised by Adrian Aichner in comp.emacs.xemacs posting] OK the problem is pretty easy to reproduce. Start up XEmacs -vanilla, then M-x load-library clearcase. Open a file in a clearcase view, then M-x clearcase-describe-current-buffer. Error message is "Wrong argument type: stringp, nil". Stack trace follows: Signaling: (wrong-type-argument stringp nil) old-call-process-internal(nil nil #<buffer "cleartoolexecution<3>"> nil "cd" "x:\\cirs\\") apply(old-call-process-internal nil nil #<buffer "cleartoolexecution<3>"> nil ("cd" "x:\\cirs\\")) call-process-internal(nil nil #<buffer "cleartoolexecution<3>"> nil "cd" "x:\\cirs\\") apply(call-process-internal nil nil #<buffer "cleartoolexecution<3>"> nil ("cd" "x:\\cirs\\")) call-process(nil nil #<buffer "cleartoolexecution<3>"> nil "cd" "x:\\cirs\\") apply(call-process nil nil #<buffer "cleartoolexecution<3>"> nil ("cd" "x:\\cirs\\")) clearcase-ct-get-command-stdout(nil "cd" "x:\\cirs\\") apply(clearcase-ct-get-command-stdout nil ("cd" "x:\\cirs\\")) clearcase-ct-blocking-call("cd" "x:\\cirs\\") clearcase-ct-cd("/cygdrive/x/cirs/") clearcase-ct-do-cleartool-command("describe" "/cygdrive/x/cirs/run_cruise.sh" unused) clearcase-describe("/cygdrive/x/cirs/run_cruise.sh") #<compiled-function nil "...(4)" [buffer-file-name clearcase-describe] 2 ("/usr/local/lib/xemacs/xemacs-packages/lisp/clearcase/clearcase.elc" . 35473) nil>() call-interactively(clearcase-describe-current-buffer) command-execute(clearcase-describe-current-buffer t) execute-extended-command(nil) call-interactively(execute-extended-command) One more thing: I noticed that you can get XEmacs to lock hard by doing the following: M-x load-library clearcase, open file in a clearcase view, M-x clearcase-mode, Options > Troubleshooting > Debug on Signal, then click on the Clearcase menu. This will yield the stack trace as above, but will also cause XEmacs to lock hard. In this state you can't even move the frame around, or even get a screenshot of the problem. It's process restart time. Should I file a separate bug report for this? Emacs: XEmacs 21.4 (patch 11) "Native Windows TTY Support (Windows)" [Lucid] (i686-pc-cygwin) of Wed Jan 8 2003 on TSUNAMI Package: clearcase.el </main/188> current state: ============== (setq system-type 'cygwin32 system-configuration "i686-pc-cygwin" emacs-version "21.4 (patch 11) \"Native Windows TTY Support (Windows)\" XEmacs Lucid" clearcase-clearcase-version-installed nil clearcase-cleartool-path nil clearcase-lt nil clearcase-v3 nil clearcase-v4 nil clearcase-v5 nil clearcase-servers-online nil clearcase-disable-tq '(cygwin32) clearcase-on-cygwin32 t clearcase-setview-root nil clearcase-suppress-vc-within-mvfs t shell-file-name "/bin/sh" )