(rmail-message-filter): Fix custom type.

lisp/ChangeLog

@@ -1,3 +1,7 @@
+2007-02-24  John Paul Wallington  <jpw@pobox.com>
+
+	* mail/rmail.el (rmail-message-filter): Fix custom type.
+
 2007-02-24  Eli Zaretskii  <eliz@gnu.org>
 
 	* startup.el (command-line): If simple.el cannot be found, proceed

lisp/mail/rmail.el

@@ -422,7 +422,7 @@ still the current message in the Rmail buffer.")
 Called with region narrowed to the message, including headers,
 before obeying `rmail-ignored-headers'."
   :group 'rmail-headers
-  :type 'function)
+  :type '(choice (const nil) function))
 
 (defcustom rmail-automatic-folder-directives nil
   "List of directives specifying where to put a message.