Trying to get the time length of a previous state

Configuration
1

I’m trying the following code to get the time length of a previous state in an Automation.

But the only result I get from the {{ trigger.from_state.for }} is ‘None’

automation 1:

  • alias: ‘Alarm’
    trigger:
    • platform: state
      entity_id: sensor.gpio17
      to: ‘0’
      action:
    • service: notify.mail
      data_template:
      title: >
      message: >
      {{ trigger.from_state.for }}

Am I using the wrong syntax?

Thanks