diff --git a/lib/fetch/index.js b/lib/fetch/index.js
index d64dd90596c..54dc6f35b18 100644
--- a/lib/fetch/index.js
+++ b/lib/fetch/index.js
@@ -2152,7 +2152,8 @@ async function httpNetworkFetch (
           } else {
             const keys = Object.keys(rawHeaders)
             for (let i = 0; i < keys.length; ++i) {
-              headersList.append(keys[i], rawHeaders[keys[i]])
+              // The header names are already in lowercase.
+              headersList.append(keys[i], rawHeaders[keys[i]], true)
             }
             // For H2, The header names are already in lowercase,
             // so we can avoid the `HeadersList#get` call here.
diff --git a/lib/fetch/request.js b/lib/fetch/request.js
index 85b8bb0df2f..f79e2bf46f0 100644
--- a/lib/fetch/request.js
+++ b/lib/fetch/request.js
@@ -471,7 +471,7 @@ class Request {
       // 3, If Content-Type is non-null and this’s headers’s header list does
       // not contain `Content-Type`, then append `Content-Type`/Content-Type to
       // this’s headers.
-      if (contentType && !this[kHeaders][kHeadersList].contains('content-type')) {
+      if (contentType && !this[kHeaders][kHeadersList].contains('content-type', true)) {
         this[kHeaders].append('content-type', contentType)
       }
     }