diff --git a/lib/Twig/Environment.php b/lib/Twig/Environment.php
index df0ffa1ad..157ebb9de 100644
--- a/lib/Twig/Environment.php
+++ b/lib/Twig/Environment.php
@@ -831,7 +831,7 @@ class Twig_Environment
             return true;
         }
 
-        return isset($this->extensionsByClass[ltrim($class, '\\')]);
+        return isset($this->extensionsByClass[$class]);
     }
 
     /**