Specified i18n.localeDetection should be undefined or a bool

Error message

Specified i18n.localeDetection should be undefined or a boolean received ${localeDetectionType}.
See more info here: https://nextjs.org/docs/messages/invalid-i18n-config

What it means

Thrown at config.ts:1351 when `i18n.localeDetection` is defined but is neither a boolean nor undefined. `localeDetection` controls automatic Accept-Header-based locale redirection; only `true`/`false`/omitted are valid.

Source

Thrown at packages/next/src/server/config.ts:1351

          `${[...duplicateLocales].join(', ')}\n` +
          `Each locale should be listed only once.\n` +
          `See more info here: https://nextjs.org/docs/messages/invalid-i18n-config`
      )
    }

    // make sure default Locale is at the front
    i18n.locales = [
      i18n.defaultLocale,
      ...i18n.locales.filter((locale) => locale !== i18n.defaultLocale),
    ]

    const localeDetectionType = typeof i18n.localeDetection

    if (
      localeDetectionType !== 'boolean' &&
      localeDetectionType !== 'undefined'
    ) {
      throw new Error(
        `Specified i18n.localeDetection should be undefined or a boolean received ${localeDetectionType}.\nSee more info here: https://nextjs.org/docs/messages/invalid-i18n-config`
      )
    }
  }

  if (result.devIndicators !== false && result.devIndicators?.position) {
    const { position } = result.devIndicators
    const allowedValues = [
      'top-left',
      'top-right',
      'bottom-left',
      'bottom-right',
    ]

    if (!allowedValues.includes(position)) {
      throw new Error(
        `Invalid "devIndicator.position" provided, expected one of ${allowedValues.join(
          ', '

View on GitHub (pinned to 0ae8c72462)

Solutions

Use a real boolean: `localeDetection: false`, or omit the key entirely.
When sourcing from env, coerce explicitly: `localeDetection: process.env.DISABLE !== '1'`.

Example fix

// before
i18n: { locales: ['en-US'], defaultLocale: 'en-US', localeDetection: 'false' }
// after
i18n: { locales: ['en-US'], defaultLocale: 'en-US', localeDetection: false }

Defensive patterns

Strategy: type-guard

Validate before calling

if (i18n.localeDetection !== undefined && typeof i18n.localeDetection !== 'boolean') {
  throw new Error('localeDetection must be boolean or undefined');
}

Type guard

function validLocaleDetection(i18n: any): boolean {
  return i18n.localeDetection === undefined || typeof i18n.localeDetection === 'boolean';
}

Prevention

Coerce env-driven values to booleans explicitly.
Omit the key rather than passing a string if you accept the default.

When it happens

Trigger: `i18n: { localeDetection: 'false' }` (string), `localeDetection: 0`, `localeDetection: null` is allowed (typeof null === 'object', which fails), `localeDetection: 'yes'`.

Common situations: Setting the value from an env var without coercing to a boolean (`process.env.DISABLE_LOCALE_DETECTION` is a string); copy-pasting config examples that quote the boolean.

Related errors

AI-assisted analysis of vercel/next.js@0ae8c72462 (2026-08-06). Data as JSON: /api/errors/15b025d90391154d. Report an issue: GitHub.