What is wrong with this automaton & scene.create?

Configuration
,
1

I’m trying to store the volume of 2 mediaplayers (only 1 in this test) when the phone is used to be able to restore it afterwards.
Something is wrong with the data_template part of the action but I cannot find out what.

  alias: 'Livingroom: FB'
  description: ''
  trigger:
  - platform: state
    entity_id: sensor.fritz_box_call_monitor
    from: idle
    to: ringing
  - platform: state
    entity_id: sensor.fritz_box_call_monitor
    from: idle
    to: dialing
  - platform: state
    entity_id: sensor.fritz_box_call_monitor
    from: idle
    to: talking
  condition:
  - condition: state
    state: playing
    entity_id: media_player.living
  action:
  - data_template:
      scene_id: beforecall
      snapshot_entities:
      - light.dl1
      entities:
        media_player.living:
          volume_level: '{{ state_attr("media_player.living", "volume_level") }}'
    service: scene.create
  - service: media_player.volume_set
    target:
      entity_id: media_player.living
    data:
      volume_level: 0.05
  mode: single

These are the step details:

params:
  domain: scene
  service: create
  service_data:
    scene_id: beforecall
    snapshot_entities:
      - light.dl1
    entities:
      media_player.living:
        volume_level: 0.08
  target: {}
running_script: false
limit: 10

Is the ‘snapshot_entities’ entry needed to make this work?