wrong-type-argument in comint mode

[Tim Johnson]

Version Info:
GNU Emacs 21.2.1 (i686-pc-linux-gnu, X toolkit, Xaw3d scroll bars) of
2002-12-12

The following defun: 
; =========================================================================
(defun rebol-send-region (beg end)
  "Send the current region to the inferior Rebol process."
  (interactive "r")
  (comint-send-region rebol-proc beg end)
  (comint-send-string rebol-proc "\n"))

;; Evaluates as expected in Xemacs, but generates the following
;;  error message from GNU emacs

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  comint-send-region("rebol" 361 401)
  rebol-send-region(361 401)
  call-interactively(rebol-send-region)

;; I can verify (I think!) that the process name is correct,
;; but it appears to me that elisp is looking for something
;; other than a string? 

Any advice, docs or solutions are welcome.
Also pointers to topics on the error message itself would
be great, I'm not entirely clear on what it is saying.

thanks
tim
-- 
Tim Johnson <tim@johnsons-web.com>
      http://www.alaska-internet-solutions.com