Index: modules/filter/filter.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/filter/filter.module,v
retrieving revision 1.347
diff -u -p -r1.347 filter.module
--- modules/filter/filter.module	28 Sep 2010 03:30:37 -0000	1.347
+++ modules/filter/filter.module	28 Sep 2010 17:41:10 -0000
@@ -678,7 +678,7 @@ function filter_list_format($format_id) 
  * @ingroup sanitization
  */
 function check_markup($text, $format_id = NULL, $langcode = '', $cache = FALSE) {
-  if (empty($format_id)) {
+  if (!isset($format_id)) {
     $format_id = filter_fallback_format();
   }
   // If the requested text format does not exist, the text cannot be filtered.