vercel/next.js · error · Error

Specified i18n.defaultLocale should be a string. See more…

Error message

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

What it means

i18n config validation in assignDefaultsAndValidate: i18n.defaultLocale was type-checked and is not a string. The default locale must be a single locale tag like 'en-US' that also appears in i18n.locales; the guard fires on the type mismatch before that membership check runs.

Solutions

1. Set i18n.defaultLocale to a string.

When it happens

Trigger: i18n.defaultLocale is not a string.

Common situations: A non-string defaultLocale in next.config i18n.

AI-assisted analysis of vercel/next.js@34433fd12e (2026-08-19). Data as JSON: /api/errors/ba5b053663b9d3a6. Report an issue: GitHub.

Appendix: source

Thrown at packages/next/src/server/config.ts:1292

      )
    }

    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`
      )
    }

    if (typeof i18n.domains !== 'undefined' && !Array.isArray(i18n.domains)) {
      throw new Error(
        `Specified i18n.domains must be an array of domain objects e.g. [ { domain: 'example.fr', defaultLocale: 'fr', locales: ['fr'] } ] received ${typeof i18n.domains}.\nSee more info here: https://nextjs.org/docs/messages/invalid-i18n-config`
      )
    }

    if (i18n.domains) {
      const invalidDomainItems = i18n.domains.filter((item) => {
        if (!item || typeof item !== 'object') return true
        if (!item.defaultLocale) return true
        if (!item.domain || typeof item.domain !== 'string') return true

        if (item.domain.includes(':')) {
          console.warn(

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