diff --git a/lisp/master.el b/lisp/master.el
index 0caf4d7963f..9151ca212d1 100644
--- a/lisp/master.el
+++ b/lisp/master.el
@@ -136,6 +136,8 @@ See `recenter'."
 (defun master-says (&optional command arg)
   "Display slave buffer and execute COMMAND with ARG in its window."
   (interactive)
+  (unless master-of
+    (error "Current buffer is not a master of any other buffer"))
   (if (null (buffer-live-p (get-buffer master-of)))
       (error "Slave buffer has disappeared")
     (let ((window  (selected-window)))