diff --git a/src/Symfony/Bridge/Twig/Extension/FormExtension.php b/src/Symfony/Bridge/Twig/Extension/FormExtension.php
index aaed8221c1..e101704efa 100644
--- a/src/Symfony/Bridge/Twig/Extension/FormExtension.php
+++ b/src/Symfony/Bridge/Twig/Extension/FormExtension.php
@@ -158,6 +158,9 @@ class FormExtension extends AbstractExtension implements InitRuntimeInterface
         return $choice->value === $selectedValue;
     }
 
+    /**
+     * @internal
+     */
     public function isRootForm(FormView $formView)
     {
         return null === $formView->parent;