diff --git a/lib/base-apis.js b/lib/base-apis.js
index b926e50a38a..e886ad1ca02 100644
--- a/lib/base-apis.js
+++ b/lib/base-apis.js
@@ -136,10 +136,12 @@ MatrixBaseApis.prototype.register = function(
     var params = {
         auth: auth
     };
-    if (username !== undefined) { params.username = username; }
-    if (password !== undefined) { params.password = password; }
-    if (bindEmail !== undefined) { params.bind_email = bindEmail; }
-    if (guestAccessToken !== undefined) { params.guest_access_token = guestAccessToken; }
+    if (username !== undefined && username !== null) { params.username = username; }
+    if (password !== undefined && password !== null) { params.password = password; }
+    if (bindEmail !== undefined && bindEmail !== null) { params.bind_email = bindEmail; }
+    if (guestAccessToken !== undefined && guestAccessToken !== null) {
+        params.guest_access_token = guestAccessToken;
+    }
 
     return this.registerRequest(params, undefined, callback);
 };