Index: wp-admin/includes/media.php
===================================================================
--- wp-admin/includes/media.php	(revision 7172)
+++ wp-admin/includes/media.php	(working copy)
@@ -451,7 +451,7 @@
 
 		$form_fields['post_content']['label'] = __('Long Description');
 
-		$thumb = wp_get_attachment_thumb_url();
+		$thumb = wp_get_attachment_thumb_url($post->ID);
 
 		$form_fields['align'] = array(
 			'label' => __('Alignment'),