diff --git a/src/base-apis.js b/src/base-apis.js
index 6491b1510..4170b13df 100644
--- a/src/base-apis.js
+++ b/src/base-apis.js
@@ -206,9 +206,6 @@ MatrixBaseApis.prototype.register = function(
         inhibitLogin = undefined;
     }
 
-    if (auth === undefined || auth === null) {
-        auth = {};
-    }
     if (sessionId) {
         auth.session = sessionId;
     }
diff --git a/src/interactive-auth.js b/src/interactive-auth.js
index 6af53ed79..215cc1350 100644
--- a/src/interactive-auth.js
+++ b/src/interactive-auth.js
@@ -104,7 +104,7 @@ const MSISDN_STAGE_TYPE = "m.login.msisdn";
  */
 export function InteractiveAuth(opts) {
     this._matrixClient = opts.matrixClient;
-    this._data = opts.authData || {};
+    this._data = opts.authData || null;
     this._requestCallback = opts.doRequest;
     this._busyChangedCallback = opts.busyChanged;
     // startAuthStage included for backwards compat