diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index 8b4d889..c566dfd 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -1809,7 +1809,7 @@
    class, as in::
 
        class Philosopher:
-           def __init_subclass__(cls, default_name, **kwargs):
+           def __init_subclass__(cls, /, default_name, **kwargs):
                super().__init_subclass__(**kwargs)
                cls.default_name = default_name