diff --git a/lib/web/fetch/request.js b/lib/web/fetch/request.js
index ca12576b1f0..60626f06a92 100644
--- a/lib/web/fetch/request.js
+++ b/lib/web/fetch/request.js
@@ -459,8 +459,9 @@ class Request {
       // 4. If headers is a Headers object, then for each header in its header
       // list, append header’s name/header’s value to this’s headers.
       if (headers instanceof HeadersList) {
-        for (const [key, val] of headers) {
-          headersList.append(key, val)
+        for (const { 0: key, 1: val } of headers) {
+          // Note: The header names are already in lowercase.
+          headersList.append(key, val, true)
         }
         // Note: Copy the `set-cookie` meta-data.
         headersList.cookies = headers.cookies