diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index 7c6d2942d..e8d34b06b 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -132,11 +132,12 @@ class RegisterRestServlet(RestServlet):
             # Set the desired user according to the AS API (which uses the
             # 'user' key not 'username'). Since this is a new addition, we'll
             # fallback to 'username' if they gave one.
-            if isinstance(body.get("user"), basestring):
-                desired_username = body["user"]
-            result = yield self._do_appservice_registration(
-                desired_username, request.args["access_token"][0]
-            )
+            desired_username = body.get("user", desired_username)
+
+            if isinstance(desired_username, basestring):
+                result = yield self._do_appservice_registration(
+                    desired_username, request.args["access_token"][0]
+                )
             defer.returnValue((200, result))  # we throw for non 200 responses
             return