Specified i18n.locales must be an array of locale strings e.

Error message

Specified i18n.locales must be an array of locale strings e.g. ["en-US", "nl-NL"] received ${typeof i18n.locales}.
See more info here: https://nextjs.org/docs/messages/invalid-i18n-config

What it means

A redundant defensive re-check at config.ts:1292 that `i18n.locales` is still an array, after the domains-validation block has run. Because the identical `!Array.isArray` check at line 1207 already guards entry into the i18n block and the block never reassigns `locales` before this point, this branch is effectively unreachable in normal flow. The message differs slightly ('must be an array of locale strings').

Source

Thrown at packages/next/src/server/config.ts:1293

          }
        }

        return hasInvalidLocale
      })

      if (invalidDomainItems.length > 0) {
        throw new Error(
          `Invalid i18n.domains values:\n${invalidDomainItems
            .map((item: any) => JSON.stringify(item))
            .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`
      )
    }

View on GitHub (pinned to 0ae8c72462)

Solutions

Treat this identically to error 200: ensure `i18n.locales` is an array of strings.
Audit any custom config plugins or `require()`-time mutations of the config object that run during normalization.

Example fix

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

Defensive patterns

Strategy: type-guard

Validate before calling

// Same as 200 — this branch is defensively redundant.
if (!Array.isArray(i18n.locales)) throw new Error('locales must be array');

Type guard

const isLocaleArray = (i18n: any): i18n is { locales: string[] } =>
  Array.isArray(i18n?.locales);

Prevention

Treat identically to error 200.
Avoid plugins that mutate `i18n.locales` mid-normalization.

When it happens

Trigger: In standard Next.js loading this is unreachable because line 1207 throws first for a non-array `locales`. It could only fire if a custom plugin or middleware mutated `i18n.locales` to a non-array between the two checks, which the core code does not do.

Common situations: Essentially never seen in practice; if you encounter it, suspect a config plugin or monkey-patch that mutates the `i18n` object during normalization.

Related errors

AI-assisted analysis of vercel/next.js@0ae8c72462 (2026-08-06). Data as JSON: /api/errors/1ba0298eb9f5c2ba. Report an issue: GitHub.