TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received undefined

Node.js · updated 2026-11-03

Thrown synchronously by Node.js core APIs (fs, path, crypto, Buffer, http) when an argument has the wrong type. The message tells you which argument, what it should be and what arrived, which is usually enough to find the bad value.

node:internal/errors:540
      throw error;
      ^

TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received undefined
    at Object.join (node:path:1253:7)
    at loadTemplates (file:///app/src/email.js:9:20) {
  code: 'ERR_INVALID_ARG_TYPE'
}

Quick fix

Look at the "Received" part: undefined almost always means a missing environment variable, config key or function argument. Find where that value comes from (often process.env.SOMETHING) and make sure it is set in this environment, or fail early with a clear message.

Read which argument it names ("path", "data", "key", "chunk") and what was received.
Follow the value back from the failing call: env variable, config file, request body, function parameter.
For env variables, check they are set in the deployed environment, not only in your local .env.
Validate required config at startup so the app fails with the variable name instead of deep inside fs or path.
For "Received an instance of Object", check you passed the field (obj.path) rather than the whole object.

typescript

// config.ts: fail at startup with the name of the missing value
function required(name: string): string {
  const v = process.env[name];
  if (!v) throw new Error(`Missing environment variable ${name}`);
  return v;
}

export const config = {
  templatesDir: required('TEMPLATES_DIR'),
  signingKey: required('SIGNING_KEY'),
};

Why it happens

A missing environment variable

process.env.X is undefined when the variable is not set where the app runs. path.join, fs.readFile and crypto.createHmac then reject it with this error.

Passing an object instead of a field

Calling fs.readFile(file) when file is { path, name } gives "Received an instance of Object". The function wanted the string inside.

Writing a number or object to a stream or response

res.write(123), res.end({ ok: true }) or Buffer.from(undefined) need a string or Buffer; serialize first with String() or JSON.stringify().

An optional value that is sometimes missing

A request field, query parameter or database column that is usually present but occasionally undefined makes the error intermittent and hard to reproduce.

Other fixes

Use a schema library (zod, valibot) to parse process.env once at startup.
In TypeScript, avoid non-null assertions (!) on env values: they turn a compile error into this runtime error.

Same error, other wording

TypeError [ERR_INVALID_ARG_TYPE]: The "data" argument must be of type string or an instance of Buffer, TypedArray, or DataView. Received undefined
TypeError [ERR_INVALID_ARG_TYPE]: The "key" argument must be of type string or an instance of ArrayBuffer, Buffer, TypedArray, DataView, KeyObject, or CryptoKey. Received undefined
TypeError [ERR_INVALID_ARG_TYPE]: The first argument must be of type string or an instance of Buffer, ArrayBuffer, or Array or an Array-like Object. Received undefined

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