diff --git a/lisp/progmodes/prolog.el b/lisp/progmodes/prolog.el
index 0e2d8d94724..d23dbad0762 100644
--- a/lisp/progmodes/prolog.el
+++ b/lisp/progmodes/prolog.el
@@ -31,6 +31,9 @@
 
 ;;; Code:
 
+(defvar comint-prompt-regexp)
+
+
 (defgroup prolog nil
   "Major mode for editing and running Prolog under Emacs."
   :group 'languages)