Triggering Alexa media player from a templated script

Configuration
1

Hopefully it’s just something really obvious that I’m missing but here goes.

I’m trying to set up a multiroom audio setup on my Amazon Echos to trigger some preset playlists which are selected via input_select and the target is also set via another input_select.

So far I have the input_selects set up without issue, and they’re setting a templated sensor to set the correct target and media type:

- platform: template
  sensors:
    audiocaster_target: # Used to determine target for audio casting
      unique_id: 65887a2b-d04f-437e-a4ae-90c421254fc5
      value_template: >
        {% if is_state("input_select.select_media_player", "Living Room") %} 
          media_player.living_room 
        {% elif is_state("input_select.select_media_player", "Kitchen") %} 
          media_player.kitchen 
        {% elif is_state("input_select.select_media_player", "Master Bedroom") %} 
          media_player.master_bedroom 
        {% elif is_state("input_select.select_media_player", "Guest Bedroom") %} 
          media_player.guest_bedroom 
        {% elif is_state("input_select.select_media_player", "Office") %} 
          media_player.office
        {% elif is_state("input_select.select_media_player", "Everywhere") %} 
          media_player.everywhere 
        {% endif %}
    audiocaster_media_type:
      unique_id: 65887a2b-d04f-437e-a4ae-90c421254fc6
      value_template: >
        {% if is_state("input_select.select_media", "Kerrang Radio") %} 
          TUNEIN 
        {% else %}
          AMAZON_MUSIC 
        {% endif %}

This gives me the correct target and media type, so no issue there.

Next I have created the following script which I intend to populate with these values and trigger from a button:

audiocaster:
  sequence:
  - service: media_player.play_media
    data_template:
      target: '{{ states("sensor.audiocaster_target") }}'
      data:
        media_content_id: '{{ states("input_select.select_media") }}'
        media_type: '{{ states("sensor.audiocaster_media_type") }}'
  mode: single
  alias: Audiocaster

But this consistently fails with:

Failed to call service script/audiocaster. extra keys not allowed @ data['target']

Any ideas what I’m doing wrong here?

2

And in addition, if I change the script to:

service: media_player.play_media
target: '{{ states("sensor.audiocaster_target") }}'
data:
  media_content_id: '{{ states("input_select.select_media") }}'
  media_type: '{{ states("sensor.audiocaster_media_type") }}'

That fails with a different issue:
Failed to call service script/audiocaster. dictionary update sequence element #0 has length 1; 2 is required