Index: wp-admin/includes/image-edit.php
===================================================================
--- wp-admin/includes/image-edit.php	(revision 18725)
+++ wp-admin/includes/image-edit.php	(working copy)
@@ -646,7 +646,7 @@
 
 		if ( $target == 'thumbnail' || $target == 'all' || $target == 'full' ) {
 			$file_url = wp_get_attachment_url($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";