diff --git a/cookbook/controller/error_pages.rst b/cookbook/controller/error_pages.rst
index d776a1575b4..99f71dd527a 100644
--- a/cookbook/controller/error_pages.rst
+++ b/cookbook/controller/error_pages.rst
@@ -50,6 +50,13 @@ end-user, create a new template located at
     </body>
     </html>
 
+.. caution::
+
+    You **must not** use ``is_granted`` in your error pages, because the
+    router runs before the firewall. If the router throws an exception (for
+    instance, when the route does not match), the error page is not behind a
+    firewall and ``app.user`` is not defined.
+
 .. tip::
 
     If you're not familiar with Twig, don't worry. Twig is a simple, powerful