vercel/next.js · error

Specified i18n.locales contains invalid values

Error message

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

What it means

Error "Specified i18n.locales contains invalid values (${invalidLocales.map(String).join(', ')}), locales must be valid locale tags provided as strings e.g. "en-US".\nSee here for list of valid language sub-tags: http://www.iana.org/assignments/language-subtag-registry/language-subtag-registry" thrown in vercel/next.js.

Solutions

1. Replace the invalid locale tags with valid BCP 47 locale strings.

When it happens

Trigger: i18n.locales contains invalid locale tags.

Common situations: Locale strings that are not valid BCP 47 tags in next.config i18n.

AI-assisted analysis of vercel/next.js@34433fd12e (2026-08-19). Data as JSON: /api/errors/0a76ff26b3f98ca3. Report an issue: GitHub.

Appendix: source

Thrown at packages/next/src/server/config.ts:1373

            .join(
              '\n'
            )}\n\ndomains value must follow format { domain: 'example.fr', defaultLocale: 'fr', locales: ['fr'] }.\nSee more info here: https://nextjs.org/docs/messages/invalid-i18n-config`
        )
      }
    }

    if (!Array.isArray(i18n.locales)) {
      throw new Error(
        `Specified i18n.locales must be an array of locale strings e.g. ["en-US", "nl-NL"] received ${typeof i18n.locales}.\nSee more info here: https://nextjs.org/docs/messages/invalid-i18n-config`
      )
    }

    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()

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