Specified i18n.defaultLocale should be included in i18n.loca

Error message

Specified i18n.defaultLocale should be included in i18n.locales.
See more info here: https://nextjs.org/docs/messages/invalid-i18n-config

What it means

Thrown at config.ts:1313 when `i18n.defaultLocale` is not a member of the `i18n.locales` array (`!i18n.locales.includes(i18n.defaultLocale)`). Next requires the default locale to be explicitly enumerated so it can resolve and promote it to the front of the list.

Source

Thrown at packages/next/src/server/config.ts:1314

    }

    const invalidLocales = i18n.locales.filter(
      (locale: any) => typeof locale !== 'string'
    )

    if (invalidLocales.length > 0) {
      throw new Error(
        `Specified i18n.locales contains invalid values (${invalidLocales
          .map(String)
          .join(
            ', '
          )}), locales must be valid locale tags provided as strings e.g. "en-US".\n` +
          `See here for list of valid language sub-tags: http://www.iana.org/assignments/language-subtag-registry/language-subtag-registry`
      )
    }

    if (!i18n.locales.includes(i18n.defaultLocale)) {
      throw new Error(
        `Specified i18n.defaultLocale should be included in i18n.locales.\nSee more info here: https://nextjs.org/docs/messages/invalid-i18n-config`
      )
    }

    const normalizedLocales = new Set()
    const duplicateLocales = new Set()

    i18n.locales.forEach((locale) => {
      const localeLower = locale.toLowerCase()
      if (normalizedLocales.has(localeLower)) {
        duplicateLocales.add(locale)
      }
      normalizedLocales.add(localeLower)
    })

    if (duplicateLocales.size > 0) {
      throw new Error(
        `Specified i18n.locales contains the following duplicate locales:\n` +

View on GitHub (pinned to 0ae8c72462)

Solutions

Add `defaultLocale` to the `locales` array exactly (same casing) as it is listed.
Standardize locale casing across both fields, e.g. always use `'en-US'`.

Example fix

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

Defensive patterns

Strategy: validation

Validate before calling

if (!i18n.locales.includes(i18n.defaultLocale)) {
  throw new Error('defaultLocale must be in locales');
}

Type guard

function defaultInLocales(i18n: { locales: string[]; defaultLocale: string }): boolean {
  return i18n.locales.includes(i18n.defaultLocale);
}

Prevention

Keep a single source of truth for locales and derive the default from it.
Match casing exactly between defaultLocale and the locales array.

When it happens

Trigger: `i18n: { locales: ['en-GB', 'fr'], defaultLocale: 'en-US' }` (default not in list); case mismatch such as `defaultLocale: 'en-us'` while locales contains `'en-US'` (the includes check is case-sensitive at this point).

Common situations: Renaming a locale in one place but not the other; using a region variant for the default that differs from the list; copying config and forgetting to add the default locale to the array.

Related errors

AI-assisted analysis of vercel/next.js@0ae8c72462 (2026-08-06). Data as JSON: /api/errors/72bcc62c30a9efc4. Report an issue: GitHub.