AttributeError: 'tuple' object has no attribute 'addCallbacks' [Twisted]

attribute-error.bash

Traceback (most recent call last):
Failure: testtools.testresult.real._StringException: Traceback (most recent call last):
  File "/home/fenn/opensource/tahoe/tahoe-lafs/venv/lib/python2.7/site-packages/twisted/internet/defer.py", line 151, in maybeDeferred
    result = f(*args, **kw)
  File "/home/fenn/opensource/tahoe/tahoe-lafs/venv/lib/python2.7/site-packages/testtools/testcase.py", line 723, in _run_test_method
    return self._get_test_method()()
  File "/home/fenn/opensource/tahoe/tahoe-lafs/src/allmydata/test/eliotutil.py", line 179, in run_with_logging
    return test_method(*args, **kwargs)
  File "/home/fenn/opensource/tahoe/tahoe-lafs/src/allmydata/test/web/test_root.py", line 111, in test_invalid
    succeeded(AfterPreprocessing(bytes, Equals(b"Invalid capability"))),
  File "/home/fenn/opensource/tahoe/tahoe-lafs/venv/lib/python2.7/site-packages/testtools/testcase.py", line 500, in assertThat
    mismatch_error = self._matchHelper(matchee, matcher, message, verbose)
  File "/home/fenn/opensource/tahoe/tahoe-lafs/venv/lib/python2.7/site-packages/testtools/testcase.py", line 551, in _matchHelper
    mismatch = matcher.match(matchee)
  File "/home/fenn/opensource/tahoe/tahoe-lafs/venv/lib/python2.7/site-packages/testtools/twistedsupport/_matchers.py", line 101, in match
    on_no_result=self._got_no_result,
  File "/home/fenn/opensource/tahoe/tahoe-lafs/venv/lib/python2.7/site-packages/testtools/twistedsupport/_deferred.py", line 86, in on_deferred_result
    deferred.addCallbacks(
AttributeError: 'tuple' object has no attribute 'addCallbacks'

test_root.py

class RenderSlashUri(SyncTestCase):
    """
    Ensure that URIs starting with /uri?uri= only accept valid
    capabilities
    """

    def setUp(self):
        self.client = object()
        self.res = URIHandler(self.client)
        super(RenderSlashUri, self).setUp()


    def test_invalid(self):
        """
        A syntactically invalid capbility results in an error.
        """
        query_args = {b"uri": [b"not a capability"]}
        response_body = render(self.res, query_args),
        self.assertThat(
            response_body,
            succeeded(AfterPreprocessing(bytes, Equals(b"Invalid capability"))),
        )