diff --git a/gtk/gtkwindow.c b/gtk/gtkwindow.c
index 755a2f8c30..af92349ca9 100644
--- a/gtk/gtkwindow.c
+++ b/gtk/gtkwindow.c
@@ -5163,7 +5163,7 @@ is_client_side_decorated (GtkWindow *window)
                         "client-side-decorated", &client_side_decorated,
                         NULL);
 
-  return client_side_decorated && window->decorated && priv->disable_client_side_decorations;
+  return client_side_decorated && window->decorated && !priv->disable_client_side_decorations;
 }
 
 static void