Cannot have null inside a string list?

[dannyroberts]

>>> import jsonobject
>>> class Foo(jsonobject.JsonObject):
...     string_list = jsonobject.ListProperty(unicode)
... 
>>> foo = Foo()
>>> foo.string_list.append(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/droberts/.virtualenvs/cchq/lib/python2.7/site-packages/jsonobject/containers.py", line 26, in append
    wrapped, unwrapped = self._wrapper.unwrap(wrapped)
  File "/Users/droberts/.virtualenvs/cchq/lib/python2.7/site-packages/jsonobject/base_properties.py", line 277, in unwrap
    self.assert_type(obj)
  File "/Users/droberts/.virtualenvs/cchq/lib/python2.7/site-packages/jsonobject/base_properties.py", line 264, in assert_type
    '{0!r} not of type {1!r}'.format(obj, self._type)
jsonobject.exceptions.BadValueError: None not of type (<type 'unicode'>, <type 'str'>)

This should work fine—or at least there should be some way to specify that None is an ok value