Specified i18n.locales should be an Array received ${typeof

Error message

Specified i18n.locales should be an Array received ${typeof i18n.locales}.
See more info here: https://nextjs.org/docs/messages/invalid-i18n-config

What it means

Thrown during Next.js config normalization when an `i18n` object is present but its `locales` field is not an Array. The validator (`Array.isArray`) runs early in the i18n block, before any locale-content checks, so a wrong-typed `locales` value aborts loading immediately. The error links to the invalid-i18n-config docs page.

Source

Thrown at packages/next/src/server/config.ts:1208

  }
  // Ensure both properties are set to the same value
  result.outputFileTracingRoot = rootDir
  dset(result, ['turbopack', 'root'], rootDir)

  setHttpClientAndAgentOptions(result || defaultConfig)

  if (result.i18n) {
    const { i18n } = result
    const i18nType = typeof i18n

    if (i18nType !== 'object') {
      throw new Error(
        `Specified i18n should be an object received ${i18nType}.\nSee more info here: https://nextjs.org/docs/messages/invalid-i18n-config`
      )
    }

    if (!Array.isArray(i18n.locales)) {
      throw new Error(
        `Specified i18n.locales should be an Array received ${typeof i18n.locales}.\nSee more info here: https://nextjs.org/docs/messages/invalid-i18n-config`
      )
    }

    if (i18n.locales.length > 100 && !silent) {
      Log.warn(
        `Received ${i18n.locales.length} i18n.locales items which exceeds the recommended max of 100.\nSee more info here: https://nextjs.org/docs/advanced-features/i18n-routing#how-does-this-work-with-static-generation`
      )
    }

    const defaultLocaleType = typeof i18n.defaultLocale

    if (!i18n.defaultLocale || defaultLocaleType !== 'string') {
      throw new Error(
        `Specified i18n.defaultLocale should be a string.\nSee more info here: https://nextjs.org/docs/messages/invalid-i18n-config`
      )
    }

View on GitHub (pinned to 0ae8c72462)

Solutions

Set `i18n.locales` to an array of locale strings, e.g. `locales: ['en-US', 'nl-NL']`.
If you have a single locale, still wrap it in an array: `locales: ['en-US']`.
Check for accidental mutation/merging of the config object that replaces the array with a scalar before Next reads it.
Remove the entire `i18n` key if you are not using the Pages Router i18n routing (App Router does not use it).

Example fix

// before
module.exports = { i18n: { locales: 'en-US', defaultLocale: 'en-US' } }
// after
module.exports = { i18n: { locales: ['en-US'], defaultLocale: 'en-US' } }

Defensive patterns

Strategy: type-guard

Validate before calling

// Before exporting config, assert the shape
const i18n = { locales: ['en-US'], defaultLocale: 'en-US' };
if (!Array.isArray(i18n.locales)) {
  throw new Error('i18n.locales must be an array');
}
module.exports = { i18n };

Type guard

function isValidI18n(i18n: unknown): i18n is { locales: unknown[]; defaultLocale: string } {
  return (
    typeof i18n === 'object' && i18n !== null &&
    Array.isArray((i18n as any).locales) &&
    typeof (i18n as any).defaultLocale === 'string'
  );
}

Prevention

Use the official NextConfig TypeScript type so the compiler rejects non-array locales.
Validate dynamic/env-driven config with a type guard before exporting.
Keep locale lists in a single typed constant and reference it from the config.

When it happens

Trigger: Setting `i18n: { locales: 'en-US' }` (a string), `i18n: { locales: { en: 'en-US' } }` (an object), `i18n: { locales: 42 }` (a number), or omitting the property entirely while still passing an `i18n` object triggers the `!Array.isArray(i18n.locales)` check at config.ts:1207.

Common situations: Developers migrating from a per-page i18n setup who write a single locale string instead of an array; copy-pasting locale config from a non-Next source; destructuring or spreading config that drops the array; TypeScript types being bypassed via `as any`.

Related errors

AI-assisted analysis of vercel/next.js@0ae8c72462 (2026-08-06). Data as JSON: /api/errors/bf85ed7bdcd70524. Report an issue: GitHub.