Next.js, blank page after calling `res.redirect()`

I had an API route and after responding to a form submit, the API called

res.redirect('/')

It worked great locally in development, but when I shipped it to Vercel, the redirect ended up to a blank page. The URL was correct, but it took a refresh to show the content.

Not sure why, but I fixed this by using res.writeHead() instead:

res.writeHead(302, { Location: '/' }).end()

The 302 Found HTTP code is a common way of performing URL redirection.