Odd mock.side_effect issue

gistfile1.py

# code under test:

class TwitterOAuth(BaseOAuth):

    def get_profile(self, access_token):
        """Retrieves the profile of the Twitter user who just authenticated.

        Requires the raw access token from the API as Twitter returns the user
        ID baked in.
        """
        # get data from twitter
        profile_url = 'http://api.twitter.com/1/users/show.json?user_id=%s'
        profile_url %= (access_token['user_id'],)

        try:
            tw_profile = json.decode(urllib2.urlopen(profile_url).read())
        except urllib2.URLError, e:
            raise ExternalServiceError("Could not contact Twitter")

        # split the name into first and (if available) last name
        name_parts = tw_profile['name'].split(' ')
        first_name = name_parts[0]
        last_name = ' '.join(name_parts[1:])

        return dict(user_id=access_token['user_id'],
                    display_name='@%s' % tw_profile['screen_name'],
                    first_name=first_name,
                    last_name=last_name,
                    profile_photo=tw_profile['profile_image_url'],
                    oauth_token=access_token['oauth_token'],
                    oauth_token_secret=access_token['oauth_token_secret'])


# test class
class TestTwitterOAuth(ApiCase):
    def setup(self):
        super(TestTwitterOAuth, self).setup()
        self.mock_request = MockRequest()

        self.provider_name = 'twitter'
        self.provider = EndUserLoginProvider.objects.get(provider=self.provider_name)

        self.urllib2_patcher = patch('bigdoor.api.oauth.twitter.urllib2')
        self.mock_urllib = self.urllib2_patcher.start()

        self.json_patcher = patch('bigdoor.api.oauth.twitter.json')
        self.mock_json = self.json_patcher.start()

        self.profile = dict(screen_name='example',
                            name='Example User',
                            profile_image_url='http://example.com/1.png')

        self.mock_json.decode.return_value = self.profile

        self.access_token = dict(user_id='1234',
                                 oauth_token=uuid4().hex,
                                 oauth_token_secret=uuid4().hex)

        self.oauth = oauth.TwitterOAuth(self.mock_request,
                                        self.publisher.app_key,
                                        self.provider_name)

    def teardown(self):
        self.urllib2_patcher.stop()
        self.json_patcher.stop()

    def test_get_profile_with_service_error(self):
        self.mock_urllib.urlopen.side_effect = urllib2.URLError('something')
        try:
            self.oauth.get_profile(self.access_token)
            assert False, "This should not have succeeded!"
        except ExternalServiceError, e:
            pass

# test results
======================================================================
ERROR: test_framework.test_oauth.TestTwitterOAuth.test_get_profile_with_service_error
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/splee/envs/publisher/lib/python2.6/site-packages/nose/case.py", line 186, in runTest
    self.test(*self.arg)
  File "/Users/splee/bigdoor/publisher/bigdoor/tests/test_framework/test_oauth.py", line 200, in test_get_profile_with_service_error
    self.oauth.get_profile(self.access_token)
  File "/Users/splee/bigdoor/publisher/bigdoor/api/oauth/twitter.py", line 22, in get_profile
    tw_profile = json.decode(urllib2.urlopen(profile_url).read())
  File "/Users/splee/envs/publisher/lib/python2.6/site-packages/mock.py", line 335, in __call__
    raise self.side_effect
URLError: <urlopen error something>

----------------------------------------------------------------------
Ran 12 tests in 0.134s

FAILED (errors=1)