diff --git a/changelog.d/4435.bugfix b/changelog.d/4435.bugfix
index 0e0535f1a3..4ea9a5df02 100644
--- a/changelog.d/4435.bugfix
+++ b/changelog.d/4435.bugfix
@@ -1 +1 @@
-Fix None guard in config.server.is_threepid_reserved
+Fix None guard in calling config.server.is_threepid_reserved
diff --git a/synapse/config/server.py b/synapse/config/server.py
index 927c54ee5b..a915bb8b64 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -429,8 +429,6 @@ def is_threepid_reserved(reserved_threepids, threepid):
     Returns:
         boolean Is the threepid undertest reserved_user
     """
-    if not threepid:
-        return False
 
     for tp in reserved_threepids:
         if (threepid['medium'] == tp['medium'] and threepid['address'] == tp['address']):
diff --git a/synapse/rest/client/v2_alpha/register.py b/synapse/rest/client/v2_alpha/register.py
index 3ab253462b..7f812b8209 100644
--- a/synapse/rest/client/v2_alpha/register.py
+++ b/synapse/rest/client/v2_alpha/register.py
@@ -416,10 +416,11 @@ class RegisterRestServlet(RestServlet):
             )
             # Necessary due to auth checks prior to the threepid being
             # written to the db
-            if is_threepid_reserved(
-                self.hs.config.mau_limits_reserved_threepids, threepid
-            ):
-                yield self.store.upsert_monthly_active_user(registered_user_id)
+            if threepid:
+                if is_threepid_reserved(
+                    self.hs.config.mau_limits_reserved_threepids, threepid
+                ):
+                    yield self.store.upsert_monthly_active_user(registered_user_id)
 
             # remember that we've now registered that user account, and with
             #  what user ID (since the user may not have specified)