diff --git a/lib/Twig/Extension/Core.php b/lib/Twig/Extension/Core.php
index a856b59b6..e8f9801cd 100644
--- a/lib/Twig/Extension/Core.php
+++ b/lib/Twig/Extension/Core.php
@@ -171,8 +171,8 @@ class Twig_Extension_Core extends Twig_Extension
         );
 
         if (function_exists('mb_get_info')) {
-            $filters['upper'] = new Twig_Filter_Function('twig_upper_filter', array('needs_environment' => true));
-            $filters['lower'] = new Twig_Filter_Function('twig_lower_filter', array('needs_environment' => true));
+            $filters[] = new Twig_SimpleFilter('upper', 'twig_upper_filter', array('needs_environment' => true));
+            $filters[] = new Twig_SimpleFilter('lower', 'twig_lower_filter', array('needs_environment' => true));
         }
 
         return $filters;