diff --git a/changelog.d/4211.bugfix b/changelog.d/4211.bugfix
new file mode 100644
index 0000000000..376f80c55a
--- /dev/null
+++ b/changelog.d/4211.bugfix
@@ -0,0 +1,2 @@
+fix start up failure when mau_limit_reserved_threepids set and db is postgres 
+
diff --git a/synapse/storage/monthly_active_users.py b/synapse/storage/monthly_active_users.py
index c353b11c9a..479e01ddc1 100644
--- a/synapse/storage/monthly_active_users.py
+++ b/synapse/storage/monthly_active_users.py
@@ -34,8 +34,9 @@ class MonthlyActiveUsersStore(SQLBaseStore):
         self.hs = hs
         self.reserved_users = ()
         # Do not add more reserved users than the total allowable number
-        self._initialise_reserved_users(
-            dbconn.cursor(),
+        self._new_transaction(
+            dbconn, "initialise_mau_threepids", [], [],
+            self._initialise_reserved_users,
             hs.config.mau_limits_reserved_threepids[:self.hs.config.max_mau_value],
         )