Device_tracker's state question

I am using the device_tracker to get infos to online time of my Kids. For that I have created the sensor:

- platform: history_stats
  name: online time kid1
  entity_id: device_tracker.desktop_kid1
  state: "home"
  type: time
  start: '{{ now().replace(hour=0, minute=0, second=0) }}'
  end: '{{ now() }}'  

The Problem is, when the state is unavailable for desktop_kid1, The time counter sensor is running like the state of device_tracker is “home”. Any idea’s

many THX