gpg-agent can't be reached

Answering myself in case someone else is stuck on this.

On Ubuntu 16.04, the default versions are:

    # gpg --version
    gpg (GnuPG) 1.4.20

and

    # gpg-agent --version
    gpg-agent (GnuPG) 2.1.11

They are not compatible. It is mandatory to use GPG2:

    # gpg2 --version
    gpg (GnuPG) 2.1.11

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