diff --git a/lisp/gnus/message.el b/lisp/gnus/message.el
index adefa0efd65..5d9473f4437 100644
--- a/lisp/gnus/message.el
+++ b/lisp/gnus/message.el
@@ -873,8 +873,8 @@ If this is nil, use `user-mail-address'.  If it is the symbol
     message-sendmail-envelope-from))
 
 (defcustom message-sendmail-extra-arguments nil
-  "Additional arguments to `sendmail-program'."
-  ;; E.g. '("-a" "account") for msmtp
+  "Additional arguments to `sendmail-program'.
+A list of strings, e.g. '("-a" "account") for msmtp."
   :version "23.1" ;; No Gnus
   :type '(repeat string)
   ;; :link '(custom-manual "(message)Mail Variables")