diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index 1740947e27..e78c5cf669 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -454,7 +454,7 @@ class RegisterRestServlet(RestServlet):
                                 # something else went wrong.
                                 break
 
-                    if self.config.register_just_use_email_for_display_name:
+                    if self.hs.config.register_just_use_email_for_display_name:
                         desired_display_name = address
                     else:
                         # XXX: a nasty heuristic to turn an email address into