Fastify is giving me an error I cannot find where im fulfilling a promise with undefined

Question

I set up a SSE endpoint on a simple Fastify project. This is the endpoint:

  app.get('/sse', async (_, res) => {
    res.raw.setHeader('Content-Type', 'text/event-stream')
    res.raw.setHeader('Cache-Control', 'no-cache')
    res.raw.setHeader('Connection', 'keep-alive')

    const updatesHashMap = {}

    const sendUpdate = async (_) => {
      const devices = await Device.getAll()
      const updates = await Update.updateData()
      console.log(new Date())

      for (const update of updates) {
        if (devices.some(device => device.id === update.deviceId)) {
          updatesHashMap[update.deviceId] = { ...update, online: !hasAMinutePassed(update.createdAt) }
        }
      }
      return res.raw.write(`data: ${JSON.stringify(updatesHashMap)}\n\n`)
    }

    const intervalId = setInterval(() => {
      sendUpdate({
        title: 'Devices',
        devices: updatesHashMap
      })
    }, 1000)

    res.raw.on('close', () => {
      clearInterval(intervalId)
    })
  })

  done()
}

In the frontend I have this connection logic with a subscriber happening

<script>
  let eventSource;

  function connectToSSE() {
    eventSource = new EventSource('/sse');

    eventSource.onmessage = (event) => {
      const data = JSON.parse(event.data);

      for (const device in data) {
        document.getElementById(`free-mem-${device}`).innerText = `${data[device].freeMem}Mb`;
        document.getElementById(`online-status-${device}`).innerText = `${data[device].online ? "Online" : "Offline" }`;
        document.getElementById(`load-${device}`).innerText = `${data[device]?.load}%`;
        document.getElementById(`uptime-${device}`).innerText = `${data[device]?.uptime}m`;
        document.getElementById(`updated-at-${device}`).innerText = data[device].createdAt;
      }
    };

    eventSource.onerror = (error) => {
      console.error('SSE Error:', error);
    };
  }

  connectToSSE();

  setInterval(() => {
    eventSource.close();
    connectToSSE();
  }, 5000);

</script>

Every 5 seconds I got an error in the logs that looks like this:

{"level":50,"time":1698804029401,"pid":159,"hostname":"82b3f69fc8b4","reqId":"req-3c","err":{"type":"FastifyError","message":"Promise may not be fulfilled with 'undefined' when statusCode is not 204","stack":"FastifyError: Promise may not be fulfilled with 'undefined' when statusCode is not 204\n    at /app/node_modules/fastify/lib/wrapThenable.js:30:30\n    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)","name":"FastifyError","code":"FST_ERR_PROMISE_NOT_FULFILLED","statusCode":500},"msg":"Promise may not be fulfilled with 'undefined' when statusCode is not 204"}

Now, I figured out that the error is related to the client disconnecting from the backend but I cannot find what is the problem with the unsubscribing proccess.

I also noticed that sometimes the client just doesnt disconnect.

This is all happening in the same Fastify app, im serving the frontend with a .eta file from within the app.

I tried already a lot of things regarding the async methods and trying not to leave chunks of code that have no return statement but so far I couldnt catch where the problem relies.