cache and json in cherrypy

Traceback (most recent call last):
  File "/Library/Python/2.7/site-packages/CherryPy-3.2.2-py2.7.egg/cherrypy/_cprequest.py", line 656, in respond
    response.body = self.handler()
  File "/Library/Python/2.7/site-packages/CherryPy-3.2.2-py2.7.egg/cherrypy/lib/jsontools.py", line 61, in json_handler
    value = cherrypy.serving.request._json_inner_handler(*args, **kwargs)
TypeError: 'NoneType' object is not callable