diff --git a/src/fetch-wrapper.ts b/src/fetch-wrapper.ts
index 7325a6ee3..fe3e7ef2b 100644
--- a/src/fetch-wrapper.ts
+++ b/src/fetch-wrapper.ts
@@ -39,6 +39,9 @@ export default function fetchWrapper(
         body: requestOptions.body,
         headers: requestOptions.headers as HeadersInit,
         redirect: requestOptions.redirect,
+        // duplex must be set if request.body is ReadableStream or Async Iterables.
+        // See https://fetch.spec.whatwg.org/#dom-requestinit-duplex.
+        ...(requestOptions.body && { duplex: "half" }),
       },
       // `requestOptions.request.agent` type is incompatible
       // see https://github.com/octokit/types.ts/pull/264