Index: wp-admin/includes/image-edit.php
===================================================================
--- wp-admin/includes/image-edit.php	(revision 23108)
+++ wp-admin/includes/image-edit.php	(working copy)
@@ -708,7 +708,7 @@
 				$return->thumbnail = $thumb_url[0];
 			} else {
 				$file_url = wp_get_attachment_url($post_id);
-				if ( $thumb = $meta['sizes']['thumbnail'] )
+				if ( !empty( $meta['sizes']['thumbnail'] ) && $thumb = $meta['sizes']['thumbnail'] )
 					$return->thumbnail = path_join( dirname($file_url), $thumb['file'] );
 				else
 					$return->thumbnail = "$file_url?w=128&h=128";