Index: src/wp-includes/link-template.php
===================================================================
--- src/wp-includes/link-template.php	(revision 34171)
+++ src/wp-includes/link-template.php	(working copy)
@@ -1182,7 +1182,7 @@
 			$preview_link = get_permalink( $post );
 		}
 
-		$query_args['preview'] = true;
+		$query_args['preview'] = 'true';
 		$preview_link = add_query_arg( $query_args, $preview_link );
 	}