reject None as the buffering argument like the C implementation does #8546
diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index adde553..cf5e9f7 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -61,7 +61,7 @@
classes. :func:`.open` uses the file's blksize (as obtained by
:func:`os.stat`) if possible.
-.. function:: open(file, mode='r', buffering=None, encoding=None, errors=None, newline=None, closefd=True)
+.. function:: open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True)
Open *file* and return a corresponding stream. If the file cannot be opened,
an :exc:`IOError` is raised.