Middleware not working when deployed

[learntheropes]

I've the following code:

vercel.json

      "config": {
        "internalServer": true,
        "serverFiles": [
          "middleware/**.js"
        ]
      }

nuxt.config.js

  serverMiddleware: ['~/middleware/response-header.js']

middleware/response-header.js

export default (req, res, next) => {
  res.setHeader("x-foo", "bar")
  res.setHeader("Cache-Control", "s-maxage=86400 stale-while-revalidate=2678000")
  next()
}

The response headers are applied when testing locally but not when deployed on vercel.

What can be the issue?