Help to identify the correct token

Hello Guys,

I'm getting a new device API setup, but I'm facing an issue with a token and need help to identify my mistake.

I have the following command response data: [{"accountId":"aa1111bb-cc2d-3333-e44f-55g666h7i888","genesis":true}]

I'm looking to get the account Id value, which, according to multiple JSON path finder should be under the token : [0].accountId

However, the value remains desperately empty :

Any idea about the possible issue ? 

I've already tried with [].accountId and accountId, but same result

Thanks in advance for any tip