diff --git a/lisp/custom.el b/lisp/custom.el
index edbbc8c1449..b7539685a89 100644
--- a/lisp/custom.el
+++ b/lisp/custom.el
@@ -306,7 +306,8 @@ The following common keywords are also meaningful.
         VALUE should be a list with the form (PACKAGE . VERSION)
         specifying that the variable was first introduced, or its
         default value was changed, in PACKAGE version VERSION.  This
-        keyword takes priority over :version.  The PACKAGE and VERSION
+        keyword takes priority over :version.  For packages which
+        are bundled with Emacs releases, the PACKAGE and VERSION
         must appear in the alist `customize-package-emacs-version-alist'.
         Since PACKAGE must be unique and the user might see it in an
         error message, a good choice is the official name of the