diff --git a/django/parts/auth/anonymoususers.py b/django/parts/auth/anonymoususers.py
index 03f294915c..b279d99d06 100644
--- a/django/parts/auth/anonymoususers.py
+++ b/django/parts/auth/anonymoususers.py
@@ -1,4 +1,6 @@
 class AnonymousUser:
+    id = None
+
     def __init__(self):
         pass
 
diff --git a/docs/authentication.txt b/docs/authentication.txt
index 9aa581cf13..63beb43031 100644
--- a/docs/authentication.txt
+++ b/docs/authentication.txt
@@ -172,6 +172,7 @@ Anonymous users
 ``django.parts.auth.anonymoususers.AnonymousUser`` is a class that implements
 the ``django.models.auth.users.User`` interface, with these differences:
 
+    * ``id`` is always ``None``.
     * ``is_anonymous()`` returns ``True`` instead of ``False``.
     * ``has_perm()`` always returns ``False``.
     * ``set_password()``, ``check_password()``, ``set_groups()`` and