Error happened when I use LiveServerTestCase to test static file

[lifeng]

I know I should use StaticLiveServerCase to test static file, and when I use it, everything is OK. But when I use LiveServerTestCase to test static file, some error occurred. Bellow is the error message:

TypeError: unsupported operand type(s) for +=: 'NoneType' and 'str'
.Traceback (most recent call last):
  File "/usr/lib/python3.3/wsgiref/handlers.py", line 137, in run
    self.result = application(self.environ, self.start_response)
  File "/home/lf/.virtualenvs/django1.7/lib/python3.3/site-packages/django/test/testcases.py", line 1011, in __call__
    return super(FSFilesHandler, self).__call__(environ, start_response)
  File "/home/lf/.virtualenvs/django1.7/lib/python3.3/site-packages/django/core/handlers/wsgi.py", line 187, in __call__
    response = self.get_response(request)
  File "/home/lf/.virtualenvs/django1.7/lib/python3.3/site-packages/django/test/testcases.py", line 994, in get_response
    return self.serve(request)
  File "/home/lf/.virtualenvs/django1.7/lib/python3.3/site-packages/django/test/testcases.py", line 1006, in serve
    return serve(request, final_rel_path, document_root=self.get_base_dir())
  File "/home/lf/.virtualenvs/django1.7/lib/python3.3/site-packages/django/views/static.py", line 50, in serve
    fullpath = os.path.join(document_root, newpath)
  File "/home/lf/.virtualenvs/django1.7/lib/python3.3/posixpath.py", line 83, in join
    path += b

The document_root is None, Is there anything wrong in ​https://github.com/django/django/blob/stable/1.7.x/django/test/testcases.py#L993

[lifeng]

Sorry, I forgot to tell my environment, it's python3.3, django1.7.2.

[Tim Graham]

It looks like you need to set settings.STATIC_ROOT to avoid that exception.