Bug Description

When sending a FormData as body with request or stream, the request fails if redirected with the following TypeError: The "string" argument must be of type string or an instance of Buffer or ArrayBuffer. Received an instance of Array

Reproducible By

In the following example a POST request is made to http://github.com, which automatically redirects to https://github.com. The issue is reproducible with any other url that triggers a redirect.

import { FormData, request } from "undici";

const body = new FormData();

body.append("hello", "world");
await request("http://github.com", {
	body,
	method: "POST",
	maxRedirections: 1,
});

Expected Behavior

Not sure about the specs but the body should be either redirected or discarded based on the redirect type, without any errors.

Logs & Screenshots

node:buffer:776
    throw new ERR_INVALID_ARG_TYPE(
          ^

TypeError [ERR_INVALID_ARG_TYPE]: The "string" argument must be of type string or an instance of Buffer or ArrayBuffer. Received an instance of Array
    at Function.byteLength (node:buffer:776:11)
    at AsyncWriter.write (D:\test\node_modules\undici\lib\dispatcher\client-h1.js:1259:24)
    at writeIterable (D:\test\node_modules\undici\lib\dispatcher\client-h1.js:1219:19) {
  code: 'ERR_INVALID_ARG_TYPE'
}

Environment

Windows 11 Home, Node v22.11.0

Additional context

Tested with various redirect urls and the issue is always reproducible