diff --git a/libxapp/xapp-display.c b/libxapp/xapp-display.c
index 3759784..0053074 100644
--- a/libxapp/xapp-display.c
+++ b/libxapp/xapp-display.c
@@ -91,7 +91,7 @@
     return;
 
   GdkScreen *screen = gtk_window_get_screen (window);
-  int active_monitor = gdk_screen_get_monitor_at_window (screen, gtk_widget_get_window(window));
+  int active_monitor = gdk_screen_get_monitor_at_window (screen, gtk_widget_get_window (GTK_WIDGET (window)));
   self->priv->num_outputs = gdk_screen_get_n_monitors (screen);
   self->priv->windows = g_new (GtkWidget *, self->priv->num_outputs);