diff --git a/modules/filter/filter.module b/modules/filter/filter.module
index 773fa80..bc31c85 100644
--- a/modules/filter/filter.module
+++ b/modules/filter/filter.module
@@ -745,9 +745,9 @@ function check_markup($text, $format_id = NULL, $langcode = '', $cache = FALSE)
 
   // Give filters the chance to escape HTML-like data such as code or formulas.
   foreach ($filters as $name => $filter) {
-    if ($filter->status && isset($filter_info[$name]['prepare callback']) && function_exists($filter_info[$name]['prepare callback'])) {
+    if ($filter->status && isset($filter_info[$name]['prepare callback'])) {
       $function = $filter_info[$name]['prepare callback'];
-      $text = $function($text, $filter, $format, $langcode, $cache, $cache_id);
+      $text = call_user_func($function, $text, $filter, $format, $langcode, $cache, $cache_id);
     }
   }