diff --git a/gdk/gdkinternals.h b/gdk/gdkinternals.h
index 1efa75e575..8949daecb8 100644
--- a/gdk/gdkinternals.h
+++ b/gdk/gdkinternals.h
@@ -203,6 +203,7 @@ struct _GdkWindow
   GdkWindowImpl *impl; /* window-system-specific delegate object */
 
   GdkWindow *parent;
+  GdkWindow *transient_for;
   GdkVisual *visual;
 
   gpointer user_data;
diff --git a/gdk/gdkwindow.c b/gdk/gdkwindow.c
index 04fb29c2bc..42085a1da0 100644
--- a/gdk/gdkwindow.c
+++ b/gdk/gdkwindow.c
@@ -10370,6 +10370,8 @@ void
 gdk_window_set_transient_for (GdkWindow *window,
 			      GdkWindow *parent)
 {
+  window->transient_for = parent;
+
   GDK_WINDOW_IMPL_GET_CLASS (window->impl)->set_transient_for (window, parent);
 }