diff --git a/fetch.js b/fetch.js
index c70622cf..d33d6a47 100644
--- a/fetch.js
+++ b/fetch.js
@@ -599,6 +599,11 @@ export function fetch(input, init) {
       Object.getOwnPropertyNames(init.headers).forEach(function(name) {
         xhr.setRequestHeader(name, normalizeValue(init.headers[name]))
       })
+      request.headers.forEach(function(value, name) {
+        if (!Object.prototype.hasOwnProperty.call(init.headers, normalizeName(name))) {
+          xhr.setRequestHeader(name, value)
+        }
+      })
     } else {
       request.headers.forEach(function(value, name) {
         xhr.setRequestHeader(name, value)