Zone valve control blueprint - help please

1

I am developing a blueprint that will turn on a heating zone control valve if any one of the smart TRVs in the zone demand heat, and turn it off only if no TRV demands heat.

The heat demand is a template binary sensor that tests whether the set temperature for a TRV (‘temperature’ attribute) is higher than the ambient temperature (‘current_temperature’ attribute).

Example heat demand sensor (seems to work)

### Guest bedroom trv heat demand
- binary_sensor:
  - name: Guest bedroom trv heat demand
    unique_id: c51617e9-da4b-9902-08ae-e623ab2d7bcc
    icon: mdi:heat-wave
    state: "{{ float(state_attr('climate.guest_bedroom_trv', 'temperature'),0) > float(state_attr('climate.guest_bedroom_trv', 'current_temperature'),99) }}" # false if either sensor is unavailable

And here is the faulty blueprint

blueprint:
  name: "Zone X"
  description: Controls a zone valve based on heat demand for all TRVs in the zone   
  domain: automation

  ### ----------------------------------------------------------------------------
  ### INPUTS
  ### ----------------------------------------------------------------------------

  input:
    zone_switch:
      name: Zone switch
      description: The switch that this automation is to control
      selector:
        entity:
          filter:
            domain: switch
          multiple: false

    heat_demand:
      name: Heat demand
      description: List all the heat demand sensors in this zone 
      selector:
        entity:
          filter:
            domain: binary_sensor
          multiple: true

mode: restart # only the latest is important 

## ----------------------------------------------------------------------------
## TRIGGERS
## ----------------------------------------------------------------------------

trigger:
  - platform: state
    entity_id: !input heat_demand

condition: []

## ----------------------------------------------------------------------------
## ACTION
## ----------------------------------------------------------------------------

action:
  - if: # True only if ALL heat demands are off 
    - condition: state
      entity_id: !input heat_demand
      state: "off"
    then:
    - service: switch.turn_off
      data: {}
      target: 
        entity_id: !input zone_switch
    else: 
    - service: switch.turn_on
      data: {}
      target: 
        entity_id: !input zone_switch

The editor (Studio Code Server) accepts the syntax but when I try to invoke the blueprint I get the cryptic error message:

Message malformed: expected a dictionary for dictionary value @ data['action'][0]['then'][0]['target']

Could soneone translate this into English please?