You have already researched for similar issues?

Yep. I may have missed something though. Hopefully not.
Closest I could come to is:
https://www.fastify.io/docs/latest/Routes/#promise-resolution

What are you trying to achieve or the steps to reproduce?

When trying to use async style routes and sendFile from fastify-static, the results properly goto the client, but a Promise may not be fulfilled with 'undefined'... is logged.

  fastify.get('/', async (request, reply) => {
    reply.sendFile('/html/index.html');
  });

What was the result you received?

err: {
      "type": "FastifyError",
      "message": "FST_ERR_PROMISE_NOT_FULLFILLED: Promise may not be fulfilled with 'undefined' when statusCode is not 204",
...
}

What did you expect?

No logged error.

Context

• node version: 12.7.0
• fastify version: 2.8.0
• os: OSX