# Node.js: “TypeError [ERR_INVALID_ARG_TYPE]: The 'path' argument must be of type string”
> TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received undefined. Find the unset value, or upgrade the tool that passes it.
Canonical: https://app369.com/fix/node-err-invalid-arg-type/
Last updated: 2026-11-08
A script or build fails with TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received undefined (or "data" must be of type string or an instance of Buffer, or "cb" must be of type function). A built-in Node.js function such as path.join, fs.readFile or Buffer.from got a value of the wrong type. The message names the argument and says what it received, which points straight at the cause.
**Short answer**
Read the last part of the message. Received undefined means a variable, environment variable or config value is not set; trace it back and set it, or fail early with a clear error. Received an instance of Object or a number means you passed the wrong thing (an object where a string or Buffer belongs, so stringify it). If the stack trace is inside a build tool such as an old react-scripts or webpack, the tool is too old for your Node.js version: upgrade it.
## **What the message received, and the fix**
1. 01 ### **Received undefined: a value that was never set**
> “A TypeError is thrown if path is not a string.” [**Node.js, path: path.dirname()**](https://nodejs.org/api/path.html) Most often an environment variable, a config key or a function argument that is missing, passed into path.join or path.resolve. Check the value before you use it, so the error names the real problem.```
const dir = process.env.UPLOAD_DIR;
if (!dir) throw new Error('UPLOAD_DIR is not set');
const file = path.join(dir, name);
```
2. 02 ### **Received an instance of Object, or a number**
> “An argument of the wrong type was passed to a Node.js API.” [**Node.js, Errors: ERR_INVALID_ARG_TYPE**](https://nodejs.org/api/errors.html) fs.writeFile, res.write and Buffer.from take a string or a Buffer, not an object or a number. Convert the value first.```
// wrong
fs.writeFileSync('out.json', data);
// right
fs.writeFileSync('out.json', JSON.stringify(data, null, 2));
res.end(String(count));
```
3. 03 ### **The "cb" argument must be of type function** The callback versions of fs functions require a callback. Use the promise version or the Sync version when you do not want one.```
import { writeFile } from 'node:fs/promises';
await writeFile('out.txt', text);
```
4. 04 ### **An old build tool on a newer Node.js** When the stack trace points into node_modules (react-scripts, webpack, an old Gulp or Babel plugin), the tool calls a Node.js API in a way newer versions reject. Upgrade that package to a release that supports your Node.js version, or run the Node.js LTS version it was built for.
Checked against the vendor’s docs on November 8, 2026: [Node.js: Errors (ERR_INVALID_ARG_TYPE)](https://nodejs.org/api/errors.html); [Node.js: path](https://nodejs.org/api/path.html).
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Related: [**Node.js “ENOENT: no such file or directory”**](https://app369.com/fix/node-enoent-no-such-file-or-directory/) · [**Node.js “Cannot find module”**](https://app369.com/fix/node-cannot-find-module/) · [**“Cannot read properties of undefined”**](https://app369.com/fix/cannot-read-properties-of-undefined/) · [**Node.js ERR_MODULE_NOT_FOUND**](https://app369.com/fix/node-err-module-not-found/)
Questions
## **More on this error**
**Why does it only fail in production or CI? **+
The environment variable or config file the code reads exists on your machine but not on the server. Add it to the hosting provider’s environment settings, and validate required variables when the app starts.
**Does a newer Node.js version cause it? **+
It can expose it. Newer Node.js versions validate arguments more strictly, so code or tools that passed the wrong type silently before now throw. Fix the call, or upgrade the tool.
**Can you fix it for me? **+
If one bug is the whole problem, the App Fix Ticket covers it: a written diagnosis first, then the fix. If the app has more going on, the Launch Check is a written check of the whole app. Each issue found is rated critical, high, medium or low, with where it is and how it was confirmed. No issues worth fixing, no charge. If the Launch Check finds nothing rated medium or higher, Simon refunds the full $450. You keep the report either way. This applies to the first 10 Launch Checks bought after October 30, 2026.
A different error?
## **Describe it and get a written quote**
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