diff --git a/source/clog-gui.lisp b/source/clog-gui.lisp
index c2a8165..c9eb165 100644
--- a/source/clog-gui.lisp
+++ b/source/clog-gui.lisp
@@ -559,7 +559,8 @@ The on-window-change clog-obj received is the new window"))
         (maphash (lambda (key value)
                    (declare (ignore key))
                    (setf new-order (z-index value))
-                   (when (>= new-order order)
+                   (when (and new-order
+                              (>= new-order order))
                      (setf order new-order)
                      (setf obj value)))
                  (windows app))))