Index: wp-admin/includes/post.php
===================================================================
--- wp-admin/includes/post.php	(revision 17170)
+++ wp-admin/includes/post.php	(working copy)
@@ -715,7 +715,7 @@
 	$mid = (int) $mid;
 
 	$meta = $wpdb->get_row( $wpdb->prepare("SELECT * FROM $wpdb->postmeta WHERE meta_id = %d", $mid) );
-	if ( is_serialized_string( $meta->meta_value ) )
+	if ( $meta && is_serialized_string( $meta->meta_value ) )
 		$meta->meta_value = maybe_unserialize( $meta->meta_value );
 	return $meta;
 }