Universal media_player state unknown


I can’t figure out why my state of my universal media_player is unknown if the state of the entity media_player.appletv is either standby or idle. It suppose the give the state off and on.
If the state of the media_player.appletv is playing or stopped, it mirrored the state to my universal media_player.
Here is the (partial) code:

- platform: universal
  name: Apple TV kijken
  state_template: >
    {% if is_state('media_player.appletv', 'standby') %}
    {% elif is_state('media_player.appletv', 'idle') %}
    {% else %}
    {{ states('media_player.appletv') }}
    {% endif %}