diff --git a/src/st/st-widget.c b/src/st/st-widget.c
index f5ca8928a..b20148906 100644
--- a/src/st/st-widget.c
+++ b/src/st/st-widget.c
@@ -579,7 +579,11 @@ get_root_theme_node (ClutterStage *stage)
 StThemeNode *
 st_widget_get_theme_node (StWidget *widget)
 {
-  StWidgetPrivate *priv = st_widget_get_instance_private (widget);
+  StWidgetPrivate *priv;
+
+  g_return_val_if_fail (ST_IS_WIDGET (widget), NULL);
+
+  priv = st_widget_get_instance_private (widget);
 
   if (priv->theme_node == NULL)
     {