diff --git a/src/matrix/net/HomeServerApi.ts b/src/matrix/net/HomeServerApi.ts
index 606514ae..62008087 100644
--- a/src/matrix/net/HomeServerApi.ts
+++ b/src/matrix/net/HomeServerApi.ts
@@ -165,19 +165,20 @@ export class HomeServerApi {
         return this._unauthedRequest("GET", this._url("/login"));
     }
 
-    register(username: string, password: string, initialDeviceDisplayName: string, auth?: Record<string, any>, inhibitLogin?: boolean , options?: IRequestOptions): IHomeServerRequest {
+    register(username: string | null, password: string, initialDeviceDisplayName: string, auth?: Record<string, any>, inhibitLogin?: boolean , options?: IRequestOptions): IHomeServerRequest {
         // todo: This is so that we disable cache-buster because it would cause the hs to respond with error
         // see https://github.com/matrix-org/synapse/issues/7722
         // need to this about the implications of this later
         const _options = options ?? {};
         Object.assign(_options, { cache: true });
+        const _username = username ?? undefined;
         return this._unauthedRequest(
             "POST",
             this._url("/register", CS_V3_PREFIX),
             undefined,
             {
                 auth,
-                username,
+                _username,
                 password,
                 initial_device_displayname: initialDeviceDisplayName,
                 inhibit_login: inhibitLogin ?? false,