KeyError: 'captcha_redirect'

How to Reproduce

While doing a POST operation on /, Trac issued an internal error.

''(please provide additional details here)''

Request parameters:

{'__FORM_TOKEN': u'bed3f2cb8e98747573f83627',
 'recaptcha_challenge_field': u'03AHJ_VutKlaYegQ5Hala6XbtuECbh5dBsKnweL8nu20pAtubV93E4ZYRXthE-SShJRSVOSdOyWz7ZD9G-LslqpcOBHHNNi05k3Zi9kra0RvTRPIw0RDsOzabjagTTKn_MARAIgGnfXkhQA_p-d3O6KOK4H2WhTNOq8zFMaJiypeiZzq2jjWmQB8I',
 'recaptcha_response_field': u'tianshou this'}

User agent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; .NET4.0E)

System Information

''System information not available''

Enabled Plugins

''Plugin information not available''

Python Traceback

Traceback (most recent call last):
  File "/usr/local/lib/python2.6/dist-packages/Trac-0.12.1-py2.6.egg/trac/web/main.py", line 511, in _dispatch_request
    dispatcher.dispatch(req)
  File "/usr/local/lib/python2.6/dist-packages/Trac-0.12.1-py2.6.egg/trac/web/main.py", line 202, in dispatch
    chosen_handler)
  File "/usr/local/lib/python2.6/dist-packages/Trac-0.12.1-py2.6.egg/trac/web/main.py", line 344, in _pre_process_request
    chosen_handler = filter_.pre_process_request(req, chosen_handler)
  File "/usr/local/lib/python2.6/dist-packages/TracSpamFilter-0.4.1dev_r10170-py2.6.egg/tracspamfilter/captcha/api.py", line 119, in pre_process_request
    del req.session['captcha_redirect']
KeyError: 'captcha_redirect'