MQTT - Dict Object has no attribute

Configuration
51

Same error but not related to Z2M. I followed this ‘tutorial’ to switch the screen on/off from my tablet: Fully Kiosk Browser
Since this is configured, and it works great, I have below error:

Logger: homeassistant.helpers.template
Source: helpers/template.py:1779
First occurred: 10:25:10 AM (75 occurrences)
Last logged: 11:03:11 AM

Template variable warning: 'dict object' has no attribute 'isScreenOn' when rendering '{{ value_json.isScreenOn }}'

According the tutorial my sensors.yaml contains:

  - platform: rest
    name: foo
    json_attributes:
      - batteryLevel
      - kioskMode
      - screenBrightness
      - motionDetectorState
      - maintenanceMode
      - appFreeMemory
      - appUsedMemory
      - totalFreeMemory
      - totalUsedMemory
      - startUrl
      - currentPage
    resource: !secret foo_rest
    value_template: '{{ value_json.isScreenOn }}'

I want to get red of this error but don’t know how