defining a service message with no return value

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I want to create a service that doesn't return anything, but returning None or an Empty message considered an error.

Test.srv

string message
---

test.py

service = rospy.Service('test', my_msgs.srv.Test, callback)

def callback(request):
    print(request.message)
    return std_srvs.srv.Empty()

results in a ServiceException, service cannot process request: handler returned invalid value: Invalid number of arguments, args should be [] args are (std_srvs.srv._Empty.Empty object at 0xffasdassa)

What's the right way of defining a service with no return value?