vercel/next.js · error · Error

Invalid fallback option

Error message

Invalid fallback option: ${fallbackField}. Fallback option must be a string, null, undefined, or false.

What it means

Schema validation in parseFallbackField: the fallback field read from the prerender manifest did not match any of the accepted shapes (string, null, false, undefined). It is a malformed-manifest signal — the value's runtime type is none of the forms the build pipeline ever writes.

Solutions

1. Set fallback in getStaticPaths to a string locale, null, undefined, or false.

When it happens

Trigger: A fallback option in routing/i18n config has an invalid type.

Common situations: Passing a non-string, non-boolean value as a fallback field in configuration.

AI-assisted analysis of vercel/next.js@0eb3775416 (2026-08-19). Data as JSON: /api/errors/846f31a0e2b313b6. Report an issue: GitHub.

Appendix: source

Thrown at packages/next/src/lib/fallback.ts:49

/**
 * Parses the fallback field from the prerender manifest.
 *
 * @param fallbackField The fallback field from the prerender manifest.
 * @returns The fallback mode.
 */
export function parseFallbackField(
  fallbackField: string | boolean | null | undefined
): FallbackMode | undefined {
  if (typeof fallbackField === 'string') {
    return FallbackMode.PRERENDER
  } else if (fallbackField === null) {
    return FallbackMode.BLOCKING_STATIC_RENDER
  } else if (fallbackField === false) {
    return FallbackMode.NOT_FOUND
  } else if (fallbackField === undefined) {
    return undefined
  } else {
    throw new Error(
      `Invalid fallback option: ${fallbackField}. Fallback option must be a string, null, undefined, or false.`
    )
  }
}

export function fallbackModeToFallbackField(
  fallback: FallbackMode,
  page: string | undefined
): string | false | null {
  switch (fallback) {
    case FallbackMode.BLOCKING_STATIC_RENDER:
      return null
    case FallbackMode.NOT_FOUND:
      return false
    case FallbackMode.PRERENDER:
      if (!page) {
        throw new Error(
          `Invariant: expected a page to be provided when fallback mode is "${fallback}"`

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