FayeJS client do not disconnect in Mocha test

I wrote an integration test with Mocha to verify a Fay websocket connection:

it('should allows clients to open ws', function (done) {
    const cli = new FayeClient(`http://localhost:${config.port}/${config.root}/ws`);
    should.exist(cli);
    const subscription = cli.subscribe('/foo', msg => {
        should.exist(msg);
        msg.should.be.deep.equal({ foo: 'bar' });
        subscription.cancel();
        cli.disconnect();
        return done();
    });
});

The subscription callback is correctly called, the message check passes but done is never called and so the test never ends. If I remove the subscription part and leave only the existence check, the test completes correctly.

How should I terminate the client?