diff --git a/lisp/org-agenda.el b/lisp/org-agenda.el
index ef41678e7..ab280fb49 100644
--- a/lisp/org-agenda.el
+++ b/lisp/org-agenda.el
@@ -2008,7 +2008,7 @@ For example, this value makes those two functions available:
 With selected entries in an agenda buffer, `B R' will call
 the custom function `set-category' on the selected entries.
 Note that functions in this alist don't need to be quoted."
-  :type 'alist
+  :type '(alist :key-type character :value-type (group function))
   :version "24.1"
   :group 'org-agenda)