为什么Selenium的回应没有sessionId？

我正在尝试运行Selenium的Firefox webdriver，并得到下面的错误。 我可以看到， response没有sessionId – 违规行是self.session_id = response['sessionId'] – 但我不知道为什么。 我已经通过以下方式运行它，并得到相同的错误：

1. Cygwin，运行nosetests
2. Cygwin直接
3. Windows，正在运行nosetests
4. Windows直接

---------------------------------------------------------------------- Traceback (most recent call last): File "C:\dev\tools\cygwin\home\207013288\dev\projects\scorpion\test\unit\test_ approve_workflows.py", line 27, in test_login 'password', userid='207013288', test=True) File "C:\dev\tools\cygwin\home\207013288\dev\projects\scorpion\src\workflows.p y", line 20, in login browser = webdriver.Firefox() File "C:\dev\sdks\Python33\lib\site-packages\selenium-2.32.0-py3.3.egg\seleniu m\webdriver\firefox\webdriver.py", line 62, in __init__ desired_capabilities=capabilities) File "C:\dev\sdks\Python33\lib\site-packages\selenium-2.32.0-py3.3.egg\seleniu m\webdriver\remote\webdriver.py", line 72, in __init__ self.start_session(desired_capabilities, browser_profile) File "C:\dev\sdks\Python33\lib\site-packages\selenium-2.32.0-py3.3.egg\seleniu m\webdriver\remote\webdriver.py", line 116, in start_session self.session_id = response['sessionId'] nose.proxy.KeyError: 'sessionId' -------------------- >> begin captured logging << -------------------- selenium.webdriver.remote.remote_connection: DEBUG: POST http://127.0.0.1:63801/ hub/session {"sessionId": null, "desiredCapabilities": {"version": "", "browserN ame": "firefox", "platform": "ANY", "javascriptEnabled": true}} --------------------- >> end captured logging << ---------------------

我之前没有使用过selenium，我不知道该从哪里出发。