That's intentional. In ConfigParser objects this exception would be raised for any other section's assignment already. You want RawConfigParser if you want to put (invalid) types as option values. See:

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