diff --git a/doc/lispref/functions.texi b/doc/lispref/functions.texi
index 77d1465c87..6d4a93b08a 100644
--- a/doc/lispref/functions.texi
+++ b/doc/lispref/functions.texi
@@ -1208,6 +1208,8 @@ Generic Functions
 Extensions for GNU Emacs Lisp}), or of one of its child classes.
 @end table
 
+Arguments marked as &optional can't be specialized on.
+
 Method definitions can make use of a new argument-list keyword,
 @code{&context}, which introduces extra specializers that test the
 environment at the time the method is run.  This keyword should appear