Template number keeps showing `Unknown`

Configuration
1 
- number:
    - name: Countdown
      unique_id: template_countdown
      unit_of_measurement: "s"
      state: >-
        {% set finishes = state_attr('timer.my_timer', 'finishes_at') %}
        {% if finishes is not none %}
          # {{ (as_datetime(finishes) - now()).total_seconds() | is_number }}
          {{ 100 | is_number }}
        {% else %}
          {{ 50 | is_number }} # Or handle the case where the timer is not running
        {% endif %}
      set_value:
        - service: timer.start
          target:
            entity_id: timer.my_timer
          data:
            duration:
              minutes: 100
      step: 1
      min: 0
      max: 240
      icon: mdi:timer

Derived from Template - Home Assistant

But my template number keeps showing Unknown, any ideas?

PS: template: is not shown because I’m splitting up my yaml files, so it is actually there and correct

2

What does this template display in the template debugger?