partial-completion-mode error

[Dan Nicolaescu]

Doing:

emacs -q
M-x partial-completion-mode RET

C-x C-f /usr/share/X11/xkb/symbols/pc TAB TAB

I get an error: Wrong type argument: sequencep, t

The error seems to be thrown by the `aref' in the following code in
PC-do-completion because `prefix' is nil:

            (if (and (not (eq mode 'help))
                     (setq prefix (try-completion (PC-chunk-after basestr skip) 
(mapcar 'list poss))))
                (let ((first t) i)
                  ;; Retain capitalization of user input even if
                  ;; completion-ignore-case is set.
                  (if (eq mode 'word)
                      (setq prefix (PC-chop-word prefix basestr)))
                  (goto-char (+ beg (length dirname)))
                  (while (and (progn
                                (setq i 0) ; index into prefix string
                                (while (< i (length prefix))
                                  (if (and (< (point) end)
                                           (eq (downcase (aref prefix i))
                                               (downcase (following-char))))
                                      ;; same char (modulo case); no action


The /usr/share/X11/xkb/symbols/ contains besides the "pc" file a "pc~"
file. If the "pc~" file is not present, no error is generated. 

Can somebody please take a look at this? 

Thanks
        --dan