Using an environment variable inside "add_ssh_keys -> fingerprints" commands

Build Environment
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Hi there. I tried use add_ssh_keys command in the way describing in topic, but job log says “There are no configured ssh keys to install”.
Documentation says it’s required step, even with using “manual ssh key instaling”. And it’s true, without fingerprint, job has failed everytime. Could anyone help with this problem please?

Part of my config:

release:
    executor: node
    steps: 
      - attach_workspace:
          at: ./
      - add_ssh_keys:
          fingerprints:
            - $GH_FNPR
      - run:
          name: Add github.com to known hosts
          command: ssh-keyscan github.com >> ~/.ssh/known_hosts

Oh, it seems like @hermant2 faced with this problem too( in this topic)