diff --git a/lib/fetch/util.js b/lib/fetch/util.js
index c8a61ac4b18..eea3905586d 100644
--- a/lib/fetch/util.js
+++ b/lib/fetch/util.js
@@ -195,7 +195,7 @@ function appendFetchMetadata (httpRequest) {
   header = httpRequest.mode
 
   //  4. Set a structured field value `Sec-Fetch-Mode`/header in r’s header list.
-  httpRequest.headersList.append('sec-fetch-mode', header)
+  httpRequest.headersList.set('sec-fetch-mode', header)
 
   //  https://w3c.github.io/webappsec-fetch-metadata/#sec-fetch-site-header
   //  TODO