diff --git a/src/matrix/net/HomeServerApi.ts b/src/matrix/net/HomeServerApi.ts
index eebc692acd..583f9f0967 100644
--- a/src/matrix/net/HomeServerApi.ts
+++ b/src/matrix/net/HomeServerApi.ts
@@ -251,7 +251,8 @@ export class HomeServerApi {
         return this._get(`/room_keys/keys/${encodeURIComponent(roomId)}/${encodeURIComponent(sessionId)}`, {version}, undefined, options);
     }
 
-    uploadRoomKeysToBackup(version: string, payload: Record<string, any>, options?: BaseRequestOptions): IHomeServerRequest {
+    uploadRoomKeysToBackup(version: string, payload: Record<string, any>, options: BaseRequestOptions = {}): IHomeServerRequest {
+        options.prefix = CS_V3_PREFIX;
         return this._put(`/room_keys/keys`, {version}, payload, options);
     }