diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index 25b1db62e..d4483c3a1 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -88,7 +88,7 @@ class RegistrationHandler(BaseHandler):
             password_hash = bcrypt.hashpw(password, bcrypt.gensalt())
 
         if localpart:
-            self.check_username(localpart)
+            yield self.check_username(localpart)
 
             user = UserID(localpart, self.hs.hostname)
             user_id = user.to_string()