Error Mini Media_player.join & SONOS

Third party integrations HADashboard
1

I get the error described below after installing the Mini Media player and adding the following card;

type: custom:mini-media-player
entity: media_player.woonkamer
hide:
  power: true
  icon: true
  source: true
speaker_group:
  platform: sonos
  show_group_count: true
  entities:
    - entity: media_player.woonkamer
      name: Sonos Living Room
    - entity: media_player.keuken
      name: Sonos Kitchen
group: true
volume_stateless: true

Failed to call service media_player/join. required key not provided @ data[‘group_members’]

Any idea what I’m missing/ doing wrong?