ServiceState not detecting output_keys

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I am using SMACH to request a Service. I want to assign a response callback that parses the Server's response and computes some output values.

Here there is the most relevant part of the code simplified for better understanding:

# hri result callback
def foo_response_cb(userdata, response):
  userdata.foo_var_out  = response.x + response.y
  return 'succeeded'

# add state to state machine
smach.StateMachine.add('FOO',
                      ServiceState('/service',
                                    myService,
                                    response_cb=foo_response_cb,
                                    output_keys=['foo_var_out']),
                      remapping={'foo_var_out':'sm_var'},
                      transitions={'succeeded':'succeeded'})

For some reason, the output values are not detected, so I get the following error:

Writing to SMACH userdata key 'foo_var_out' but the only keys that were declared as output from this state were: ().

Which is the same error I get if I do not add any output key at all.

Can anybody give some hint of what may be happening?