Callback not being fired

[ffxsam]

logger.log(
  'info',
  'Callback test',
  {
    reqid: 'aaaaaa-bbbbb-23321',
    details: {
      name: 'test',
    },
  },
  () => {
    console.log('callback fired');
  }
);

When I make this call, the callback function is not getting fired for some reason. I looked at the winston-papertrail code, and I can't find anything wrong with it. It passes the callback to stream.write() which looks good.