BadYieldError in thrift py:tornado server

Happens with a tornado server and a regular python client. The call completes successfully on the client side but throws this exception on the server side

Traceback (most recent call last):
File "/Users/ravithangavel/workspace/fastTrackService/src/hack.py", line 177, in handle_stream
yield self._processor.process(iprot, oprot)
File "/Users/ravithangavel/.virtualenvs/tutorial/lib/python2.7/site-packages/tornado/gen.py", line 1008, in run
value = future.result()
File "/Users/ravithangavel/.virtualenvs/tutorial/lib/python2.7/site-packages/tornado/concurrent.py", line 232, in result
raise_exc_info(self._exc_info)
File "/Users/ravithangavel/.virtualenvs/tutorial/lib/python2.7/site-packages/tornado/gen.py", line 1090, in handle_yield
self.future = convert_yielded(yielded)
File "/Users/ravithangavel/.virtualenvs/tutorial/lib/python2.7/site-packages/singledispatch.py", line 210, in wrapper
return dispatch(args[0]._class_)(*args, **kw)
File "/Users/ravithangavel/.virtualenvs/tutorial/lib/python2.7/site-packages/tornado/gen.py", line 1222, in convert_yielded
raise BadYieldError("yielded unknown object %r" % (yielded,))