Index: wp-includes/widgets.php
===================================================================
--- wp-includes/widgets.php	(revision 15570)
+++ wp-includes/widgets.php	(working copy)
@@ -854,6 +854,7 @@
 	}
 
 	$sidebars_widgets = wp_get_sidebars_widgets();
+	if ( !count( $sidebars_widgets ) ) return false;
 
 	if ( empty($wp_registered_sidebars[$index]) || !array_key_exists($index, $sidebars_widgets) || !is_array($sidebars_widgets[$index]) || empty($sidebars_widgets[$index]) )
 		return false;