diff --git a/lib/adapters/fetch.js b/lib/adapters/fetch.js
index e52fa38..0bd5bfc 100644
--- a/lib/adapters/fetch.js
+++ b/lib/adapters/fetch.js
@@ -69,7 +69,11 @@ const getBodyLength = async (body) => {
   }
 
   if(utils.isSpecCompliantForm(body)) {
-    return (await new Request(body).arrayBuffer()).byteLength;
+    const _request = new Request(platform.origin, {
+      method: 'POST',
+      body,
+    });
+    return (await _request.arrayBuffer()).byteLength;
   }
 
   if(utils.isArrayBufferView(body) || utils.isArrayBuffer(body)) {