What does it mean when io.TextIOBase.encoding is None

[Rocco]

When using nose to run unittests, i discovered that depending on nose' command line parameters, sys.stdout.encoding can be None.

I wonder what that means for an io.TextIOBase object.
Is None a valid value for 'encoding'?
Is None equivalent to 'ascii' in this case?
If None is a valid value for io.TextIOBase.encoding, then why is it not a valid value for bytes.encode()?

Regards,
Rocco