None is an invalid value in the configparser. It only accepts strings. See:

>>> cp = ConfigParser()
>>> cp['asd'] = {'a': None}
Traceback (most recent call last):
...
TypeError: option values must be strings



The DEFAULT section was an omission which is now fixed. You can use a RawConfigParser if you want to put invalid types as option values:

>>> rcp = RawConfigParser()
>>> rcp['asd'] = {'a': None}
>>>