diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index 3abfe3547..c6f4680a7 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -368,8 +368,8 @@ class RegisterRestServlet(RestServlet):
         if auth_result:
             for login_type in [LoginType.EMAIL_IDENTITY, LoginType.MSISDN]:
                 if login_type in auth_result:
-                    medium = auth_result[login_type].threepid['medium']
-                    address = auth_result[login_type].threepid['address']
+                    medium = auth_result[login_type]['medium']
+                    address = auth_result[login_type]['address']
 
                     if not check_3pid_allowed(self.hs, medium, address):
                         raise SynapseError(