vercel/next.js · error · Error

Specified pageExtensions is not an array of strings, found

Error message

Specified pageExtensions is not an array of strings, found "${ext}" of type "${typeof ext}". Please update this config or remove it.

What it means

Per-element validation in assignDefaultsAndValidate: pageExtensions is an array, but iterating its entries found an element whose typeof is not string (a number, null, object, etc.). The offending element and its type are echoed; every extension in the list must be a string like 'tsx'.

Solutions

1. Make every pageExtensions entry a string.

When it happens

Trigger: An entry in pageExtensions is not a string.

Common situations: A number or object inside the pageExtensions array in next.config.

AI-assisted analysis of vercel/next.js@34433fd12e (2026-08-19). Data as JSON: /api/errors/979666ecebf6fb3f. Report an issue: GitHub.

Appendix: source

Thrown at packages/next/src/server/config.ts:417

        }
      }

      if (key === 'pageExtensions') {
        if (!Array.isArray(value)) {
          throw new Error(
            `Specified pageExtensions is not an array of strings, found "${value}". Please update this config or remove it.`
          )
        }

        if (!value.length) {
          throw new Error(
            `Specified pageExtensions is an empty array. Please update it with the relevant extensions or remove it.`
          )
        }

        value.forEach((ext) => {
          if (typeof ext !== 'string') {
            throw new Error(
              `Specified pageExtensions is not an array of strings, found "${ext}" of type "${typeof ext}". Please update this config or remove it.`
            )
          }
        })
      }

      const defaultValue = (defaultConfig as Record<string, unknown>)[key]

      if (
        !!value &&
        value.constructor === Object &&
        typeof defaultValue === 'object'
      ) {
        currentConfig[key] = {
          ...defaultValue,
          ...Object.keys(value).reduce<any>((c, k) => {
            const v = value[k]
            if (v !== undefined && v !== null) {

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