diff --git a/src/matrix/net/HomeServerApi.js b/src/matrix/net/HomeServerApi.js
index 918987c5..99299190 100644
--- a/src/matrix/net/HomeServerApi.js
+++ b/src/matrix/net/HomeServerApi.js
@@ -55,13 +55,13 @@ export class HomeServerApi {
             .join("&");
         url = `${url}?${queryString}`;
         let bodyString;
-        const headers = new Headers();
+        const headers = new Map();
         if (this._accessToken) {
-            headers.append("Authorization", `Bearer ${this._accessToken}`);
+            headers.set("Authorization", `Bearer ${this._accessToken}`);
         }
-        headers.append("Accept", "application/json");
+        headers.set("Accept", "application/json");
         if (body) {
-            headers.append("Content-Type", "application/json");
+            headers.set("Content-Type", "application/json");
             bodyString = JSON.stringify(body);
         }
         const requestResult = this._requestFn(url, {
diff --git a/src/matrix/net/request/fetch.js b/src/matrix/net/request/fetch.js
index bc9ea8b7..886e8195 100644
--- a/src/matrix/net/request/fetch.js
+++ b/src/matrix/net/request/fetch.js
@@ -44,6 +44,13 @@ export function fetchRequest(url, options) {
         referrer: "no-referrer",
         cache: "no-cache",
     });
+    if (options.headers) {
+        const headers = new Headers();
+        for(const [name, value] of options.headers.entries()) {
+            headers.append(name, value);
+        }
+        options.headers = headers;
+    }
     const promise = fetch(url, options).then(async response => {
         const {status} = response;
         const body = await response.json();