diff --git a/lib/xmlhttprequest.js b/lib/xmlhttprequest.js
index ba631bb..adfe94d 100644
--- a/lib/xmlhttprequest.js
+++ b/lib/xmlhttprequest.js
@@ -339,7 +339,11 @@ class XMLHttpRequest extends XMLHttpRequestEventTarget {
       client.setHeader(key, value);
     });
     _readyStateChange.call(this, XMLHttpRequest.HEADERS_RECEIVED);
-    if (typeof body === 'string' || body instanceof FormData) {
+    if (body instanceof Uint8Array || body instanceof ArrayBuffer) {
+      body = Buffer.from(body);
+    }
+    if (typeof body === 'string' || body instanceof Buffer || body instanceof FormData) {
+      client.setHeader('Content-Length', Buffer.isBuffer(body) ? body.length : Buffer.byteLength(body));
       client.on('socket', _setDispatchProgressEvents.bind(this.upload));
       client.write(body);
     }