vercel/next.js · error · Error

Specified distDir is not a string, found type

Error message

Specified distDir is not a string, found type "${typeof value}"

What it means

Config validation in assignDefaultsAndValidate: while reducing over the user's config keys, the distDir key was checked with typeof and found to be a non-string. distDir must be a directory path string; the received type is named so the offending next.config.js value can be corrected.

Solutions

1. Set distDir to a string in next.config.js.

When it happens

Trigger: distDir in next.config is not a string.

Common situations: Setting distDir to a number, boolean, or object in next.config.

AI-assisted analysis of vercel/next.js@34433fd12e (2026-08-19). Data as JSON: /api/errors/2b7907df301ee2d1. Report an issue: GitHub.

Appendix: source

Thrown at packages/next/src/server/config.ts:380

      )
    }
    if (typeof userConfig.trailingSlash === 'undefined') {
      userConfig.trailingSlash = (userConfig as any).exportTrailingSlash
    }
    delete (userConfig as any).exportTrailingSlash
  }

  const config = Object.keys(userConfig).reduce<{ [key: string]: any }>(
    (currentConfig, key) => {
      const value = (userConfig as any)[key]

      if (value === undefined || value === null) {
        return currentConfig
      }

      if (key === 'distDir') {
        if (typeof value !== 'string') {
          throw new Error(
            `Specified distDir is not a string, found type "${typeof value}"`
          )
        }
        const userDistDir = value.trim()

        // don't allow public as the distDir as this is a reserved folder for
        // public files
        if (userDistDir === 'public') {
          throw new Error(
            `The 'public' directory is reserved in Next.js and can not be set as the 'distDir'. https://nextjs.org/docs/messages/can-not-output-to-public`
          )
        }
        // make sure distDir isn't an empty string as it can result in the provided
        // directory being deleted in development mode
        if (userDistDir.length === 0) {
          throw new Error(
            `Invalid distDir provided, distDir can not be an empty string. Please remove this config or set it to undefined`
          )

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