Index: wp-includes/revision.php
===================================================================
--- wp-includes/revision.php	(revision 23862)
+++ wp-includes/revision.php	(working copy)
@@ -523,6 +523,9 @@
 		return $value;
 
 	$preview = wp_get_post_autosave( $post->ID );
+	if ( ! is_object( $preview ) )
+		return $value;
+
 	return get_post_meta( $preview->ID, $meta_key, $single );
 }