This is a potentially annoying incompatibility between platforms.
Under Linux:

>>> import os
>>> print(os.device_encoding(1000))
None

And under Windows:

>>> print(os.device_encoding(1000))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 9] Bad file descriptor

This may be related to the following lines in posixmodule.c:

if (!_PyVerify_fd(fd))
	return posix_error();

Right, None is clearly the right result.  Fixed in revision 70581

Thanks!