diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index 6a97c06110..a36fb6b3bd 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -809,7 +809,7 @@ class Auth(object):
             elif threepid:
                 # If the user does not exist yet, but is signing up with a
                 # reserved threepid then pass auth check
-                if is_threepid_reserved(threepid):
+                if self.store.is_threepid_reserved(threepid):
                     return
             # Else if there is no room in the MAU bucket, bail
             current_mau = yield self.store.get_monthly_active_count()