[10.0.0][i18n][notFound][Vercel] undefined locale and locales in getStaticProps on custom 404-page

[rokinsky]

Bug report

Describe the bug

I use the notFound and i18n features on SSG pages with enabled fallback (blocking strategy). Variables locale and locales are undefined in getStaticProps context when notFound occurs. This happens only when an app is deployed to Vercel.

To Reproduce

Repository: next.js-bug-i18n-not-found
Steps to reproduce the behavior:

1. Deploy the app to Vercel
2. Go to the website, e.g. https://next-js-bug-i18n-not-found.vercel.app
3. See

{
  "is404": true,
  "locale": "",
  "locales": []
}

Expected behavior

Displaying

{
  "is404": true,
  "locale": "en",
  "locales": [
    "en",
    "nl-nl",
    "de-de"
  ]
}

Screenshots

N/A

System information

• OS: macOS
• Browser (if applies): chrome
• Version of Next.js: 10.0.0
• Version of Node.js: v10.22.1

Additional context

Works fine locally in dev and prod modes.

[balazsorban44]

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