diff --git a/src/compositor/mutter/compositor-mutter.c b/src/compositor/mutter/compositor-mutter.c
index 65c1cd357..8efc7af6b 100644
--- a/src/compositor/mutter/compositor-mutter.c
+++ b/src/compositor/mutter/compositor-mutter.c
@@ -1221,7 +1221,7 @@ add_win (MetaWindow *window)
   meta_verbose ("added 0x%x (%p) type:", (guint)top_window, cw);
 
   /* Hang our compositor window state off the MetaWindow for fast retrieval */
-  meta_window_set_compositor_private (window, cw);
+  meta_window_set_compositor_private (window, G_OBJECT (cw));
 
   /*
    * Add this to the list at the top of the stack before it is mapped so that
@@ -1745,7 +1745,7 @@ clutter_cmp_remove_window (MetaCompositor *compositor,
   MutterWindow         *cw     = NULL;
 
   DEBUG_TRACE ("clutter_cmp_remove_window\n");
-  cw = meta_window_get_compositor_private (window);
+  cw = MUTTER_WINDOW (meta_window_get_compositor_private (window));
   if (!cw)
     return;
 
@@ -1874,7 +1874,7 @@ static void
 clutter_cmp_map_window (MetaCompositor *compositor, MetaWindow *window)
 {
 #ifdef HAVE_COMPOSITE_EXTENSIONS
-  MutterWindow *cw = meta_window_get_compositor_private (window);
+  MutterWindow *cw = MUTTER_WINDOW (meta_window_get_compositor_private (window));
   DEBUG_TRACE ("clutter_cmp_map_window\n");
   if (!cw)
     return;
@@ -1887,7 +1887,7 @@ static void
 clutter_cmp_unmap_window (MetaCompositor *compositor, MetaWindow *window)
 {
 #ifdef HAVE_COMPOSITE_EXTENSIONS
-  MutterWindow *cw = meta_window_get_compositor_private (window);
+  MutterWindow *cw = MUTTER_WINDOW (meta_window_get_compositor_private (window));
   DEBUG_TRACE ("clutter_cmp_unmap_window\n");
   if (!cw)
     return;
@@ -1903,7 +1903,7 @@ clutter_cmp_minimize_window (MetaCompositor *compositor,
 			     MetaRectangle  *icon_rect)
 {
 #ifdef HAVE_COMPOSITE_EXTENSIONS
-  MutterWindow	 *cw = meta_window_get_compositor_private (window);
+  MutterWindow	 *cw = MUTTER_WINDOW (meta_window_get_compositor_private (window));
   MetaScreen	 *screen = meta_window_get_screen (window);
   MetaCompScreen *info = meta_screen_get_compositor_data (screen);
 
@@ -1939,7 +1939,7 @@ clutter_cmp_unminimize_window (MetaCompositor *compositor,
 {
 #ifdef HAVE_COMPOSITE_EXTENSIONS
 #if 0
-  MutterWindow	 *cw = meta_window_get_compositor_private (window);
+  MutterWindow	 *cw = MUTTER_WINDOW (meta_window_get_compositor_private (window));
   MetaScreen	 *screen = meta_window_get_screen (window);
   MetaCompScreen *info = meta_screen_get_compositor_data (screen);
 
@@ -1963,7 +1963,7 @@ clutter_cmp_unminimize_window (MetaCompositor *compositor,
       cw->priv->minimize_in_progress--;
     }
 #else
-  MutterWindow	 *cw = meta_window_get_compositor_private (window);
+  MutterWindow	 *cw = MUTTER_WINDOW (meta_window_get_compositor_private (window));
   DEBUG_TRACE ("clutter_cmp_unminimize_window\n");
   if (!cw)
     return;
@@ -1980,7 +1980,7 @@ clutter_cmp_maximize_window (MetaCompositor *compositor,
 			     MetaRectangle  *rect)
 {
 #ifdef HAVE_COMPOSITE_EXTENSIONS
-  MutterWindow	 *cw = meta_window_get_compositor_private (window);
+  MutterWindow	 *cw = MUTTER_WINDOW (meta_window_get_compositor_private (window));
   MetaScreen	 *screen = meta_window_get_screen (window);
   MetaCompScreen *info = meta_screen_get_compositor_data (screen);
 
@@ -2010,7 +2010,7 @@ clutter_cmp_unmaximize_window (MetaCompositor *compositor,
 			       MetaRectangle  *rect)
 {
 #ifdef HAVE_COMPOSITE_EXTENSIONS
-  MutterWindow	 *cw = meta_window_get_compositor_private (window);
+  MutterWindow	 *cw = MUTTER_WINDOW (meta_window_get_compositor_private (window));
   MetaScreen	 *screen = meta_window_get_screen (window);
   MetaCompScreen *info = meta_screen_get_compositor_data (screen);
 
@@ -2169,7 +2169,7 @@ clutter_cmp_sync_stack (MetaCompositor *compositor,
   for (tmp = stack; tmp != NULL; tmp = tmp->next)
     {
       MetaWindow    *window = tmp->data;
-      MutterWindow  *cw = window->compositor_private;
+      MutterWindow  *cw = MUTTER_WINDOW (meta_window_get_compositor_private (window));
 
       if (!cw)
 	{
@@ -2191,7 +2191,7 @@ clutter_cmp_set_window_hidden (MetaCompositor *compositor,
 			       MetaWindow     *window,
 			       gboolean	       hidden)
 {
-  MutterWindow *cw = window->compositor_private;
+  MutterWindow *cw = MUTTER_WINDOW (meta_window_get_compositor_private (window));
   MetaCompScreen *info = meta_screen_get_compositor_data (screen);
 
   DEBUG_TRACE ("clutter_cmp_set_window_hidden\n");
@@ -2219,7 +2219,7 @@ clutter_cmp_sync_window_geometry (MetaCompositor *compositor,
 				  MetaWindow *window)
 {
 #ifdef HAVE_COMPOSITE_EXTENSIONS
-  MutterWindow	 *cw = meta_window_get_compositor_private (window);
+  MutterWindow	 *cw = MUTTER_WINDOW (meta_window_get_compositor_private (window));
   MetaScreen	 *screen = meta_window_get_screen (window);
   MetaCompScreen *info = meta_screen_get_compositor_data (screen);
 
diff --git a/src/core/window-private.h b/src/core/window-private.h
index efdb3476e..5255b774b 100644
--- a/src/core/window-private.h
+++ b/src/core/window-private.h
@@ -360,7 +360,7 @@ struct _MetaWindow
   MetaGroup *group;
 
 #ifdef HAVE_COMPOSITE_EXTENSIONS
-  void *compositor_private;
+  GObject *compositor_private;
 #endif
 };
 
diff --git a/src/core/window.c b/src/core/window.c
index e4d51cc62..a9a0d9099 100644
--- a/src/core/window.c
+++ b/src/core/window.c
@@ -8707,7 +8707,15 @@ meta_window_get_description (MetaWindow *window)
   return window->desc;
 }
 
-void *
+/**
+ * meta_window_get_compositor_private:
+ * @window: a #MetaWindow
+ *
+ * Gets the compositor's wrapper object for @window.
+ *
+ * Return value: (transfer none): the wrapper object.
+ **/
+GObject *
 meta_window_get_compositor_private (MetaWindow *window)
 {
   if (!window)
@@ -8716,7 +8724,7 @@ meta_window_get_compositor_private (MetaWindow *window)
 }
 
 void
-meta_window_set_compositor_private (MetaWindow *window, void *priv)
+meta_window_set_compositor_private (MetaWindow *window, GObject *priv)
 {
   if (!window)
     return;
diff --git a/src/include/window.h b/src/include/window.h
index b99816dca..1ee2af647 100644
--- a/src/include/window.h
+++ b/src/include/window.h
@@ -92,8 +92,8 @@ void meta_window_change_workspace_by_index (MetaWindow *window,
                                             gint        space_index,
                                             gboolean    append,
                                             guint32     timestamp);
-void *meta_window_get_compositor_private (MetaWindow *window);
-void meta_window_set_compositor_private (MetaWindow *window, void *priv);
+GObject *meta_window_get_compositor_private (MetaWindow *window);
+void meta_window_set_compositor_private (MetaWindow *window, GObject *priv);
 void meta_window_configure_notify (MetaWindow *window, XConfigureEvent *event);
 const char *meta_window_get_role (MetaWindow *window);