hub.challenge missing

[reddyse]

When I try to "unsubscribe" the topic from an external client (request pasted below). I am not getting the hub.challenge field from the Hub, it works perfectly fine for "subscribe"

requests.post('http://localhost:5000/api/hub', data = {'hub.callback':'http://localhost:4449','hub.mode':'unsubscribe','hub.topic':"my_topic",'hub.secret':'secret1','hub.events':'open-patient-chart','hub.lease_seconds':'3600'})