Mod_python error: "PythonHandler cldispatcher"

Traceback (most recent call last):

  File "/usr/lib/python2.4/site-packages/mod_python/apache.py", line 299, in HandlerDispatch
    result = object(req)

  File "/home/httpd/villanimaterialidirecupero.it/cldispatcher.py", line 83, in handler
    return handlerfunc(req)

  File "/home/httpd/villanimaterialidirecupero.it/clresources/product.py", line 102, in handler
    req.write(str(HTMLResource(req)))

  File "/home/httpd/villanimaterialidirecupero.it/clresources/product.py", line 54, in __str__
    item = self._uri_method.handle()

  File "/home/httpd/villanimaterialidirecupero.it/clresources/product.py", line 43, in handle
    assert(len(listaProdotti) == 1)

AssertionError