500 Internal Server Error

The server encountered an unexpected condition which prevented it from fulfilling the request.

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/cherrypy/_cprequest.py", line 656, in respond
    response.body = self.handler()
  File "/usr/local/lib/python2.7/dist-packages/cherrypy/lib/encoding.py", line 188, in __call__
    self.body = self.oldhandler(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/cherrypy/_cpdispatch.py", line 34, in __call__
    return self.callable(*self.args, **self.kwargs)
  File "/usr/local/share/neuron/0.2/ncm/controllers.py", line 129, in default
    return NeuronServer(site).execute(nl, **data)
  File "/usr/local/share/neuron/0.2/ncm/controllers.py", line 317, in execute
    if not NeuronMapper(self.site, nl).call():
  File "/usr/local/share/neuron/0.2/ncm/controllers.py", line 451, in call
    handler(**args)
  File "/usr/local/share/neuron/0.2/ncm/modules/NShop/controllers.py", line 1412, in add_to_cart
    self.site.redirect_back()
  File "/usr/local/share/neuron/0.2/ncm/controllers.py", line 217, in redirect_back
    raise cherrypy.HTTPRedirect(cherrypy.session.get("__referer"))
  File "/usr/local/lib/python2.7/dist-packages/cherrypy/_cperror.py", line 196, in __init__
    for url in urls:
TypeError: 'NoneType' object is not iterable
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