Waze_travel_time.get_travel_times origin not accepting templates

1 
- variables:
    coords: |-
      {%- if is_state("person.bart_simpson", "home") -%}
        {%- set latitude = state_attr('zone.home', 'latitude') -%}
        {%- set longitude = state_attr('zone.home', 'longitude') -%}
      {%- else  -%}
        {%- set latitude = state_attr(state_attr('person.bart_simpson', 'source'), 'latitude') -%}
        {%- set longitude = state_attr(state_attr('person.bart_simpson', 'source'), 'longitude') -%}
      {%- endif -%}
      {{ latitude ~ "," ~ longitude }}

- action: waze_travel_time.get_travel_times
  metadata: {}
  data:
    region: eu
    units: metric
    vehicle_type: car
    origin: "{{ coords }}"
    destination: "{{ repeat.item.location }}"
    realtime: true
  response_variable: travel

{{ repeat.item.location }} works fine and it’s coming from calendar.get_events.
If I use "{{ coords }}" as the origin, I get the following error:

Executed: 21 November 2024 at 19:38:53
Error: expected str for dictionary value @ data['origin']
Result:

params:
  domain: waze_travel_time
  service: get_travel_times
  service_data:
    region: eu
    units: metric
    vehicle_type: car
    origin:
      - 11.111
      - -1.111
    destination: Some random street
    realtime: true
  target: {}
running_script: false

How can I force it to be a string ?