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

diff --git a/lisp/mail/rmail.el b/lisp/mail/rmail.el
index 5c0412fa2becf1093546ae977c7b21c52b6a8ade..5e534b2d47d8026c588b89144265fc2295270e04 100644 (file)
--- a/lisp/mail/rmail.el
+++ b/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.