Custom component sensor state translations

Hi,
I am trying to figure out how to translate sensor state properly in my custom component.

I have a timer value that should return timestamp if it has been set or localized string informing if time has not been set. I have it currently like this in translations/en.json:

    "state": {
      "not_set": "Not set"
    },

And it is working! Timer gets correct translated string value.
What is not working so well - when I try to validate using hassfest, I get:

Error: R] [TRANSLATIONS] Invalid translations/en.json: extra keys not allowed @ data['state']. Got {'not_set': 'Not set'}

What am I doing wrong?