diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index 41a13ece7..9ada3ca5c 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -388,7 +388,7 @@ class RegisterRestServlet(RestServlet):
                     )
                 ):
                     address = auth_result[login_type]['address']
-                    desired_username = address.replace('@', '.').lower()
+                    desired_username = address.replace('@', '-').lower()
 
         if desired_username is not None:
             yield self.registration_handler.check_username(