diff --git a/changelog.d/3681.bugfix b/changelog.d/3681.bugfix
new file mode 100644
index 0000000000..d18a69cd0c
--- /dev/null
+++ b/changelog.d/3681.bugfix
@@ -0,0 +1 @@
+Fixes test_reap_monthly_active_users so it passes under postgres
diff --git a/synapse/storage/monthly_active_users.py b/synapse/storage/monthly_active_users.py
index d47dcef3a0..9a6e699386 100644
--- a/synapse/storage/monthly_active_users.py
+++ b/synapse/storage/monthly_active_users.py
@@ -64,23 +64,27 @@ class MonthlyActiveUsersStore(SQLBaseStore):
             Deferred[]
         """
         def _reap_users(txn):
+            # Purge stale users
 
             thirty_days_ago = (
                 int(self._clock.time_msec()) - (1000 * 60 * 60 * 24 * 30)
             )
-            # Purge stale users
-
-            # questionmarks is a hack to overcome sqlite not supporting
-            # tuples in 'WHERE IN %s'
-            questionmarks = '?' * len(self.reserved_users)
             query_args = [thirty_days_ago]
-            query_args.extend(self.reserved_users)
+            base_sql = "DELETE FROM monthly_active_users WHERE timestamp < ?"
 
-            sql = """
-                DELETE FROM monthly_active_users
-                WHERE timestamp < ?
-                AND user_id NOT IN ({})
-                """.format(','.join(questionmarks))
+            # Need if/else since 'AND user_id NOT IN ({})' fails on Postgres
+            # when len(reserved_users) == 0. Works fine on sqlite.
+            if len(self.reserved_users) > 0:
+                # questionmarks is a hack to overcome sqlite not supporting
+                # tuples in 'WHERE IN %s'
+                questionmarks = '?' * len(self.reserved_users)
+
+                query_args.extend(self.reserved_users)
+                sql = base_sql + """ AND user_id NOT IN ({})""".format(
+                    ','.join(questionmarks)
+                )
+            else:
+                sql = base_sql
 
             txn.execute(sql, query_args)
 
@@ -93,16 +97,24 @@ class MonthlyActiveUsersStore(SQLBaseStore):
             # negative LIMIT values. So there is no way to write it that both can
             # support
             query_args = [self.hs.config.max_mau_value]
-            query_args.extend(self.reserved_users)
-            sql = """
+
+            base_sql = """
                 DELETE FROM monthly_active_users
                 WHERE user_id NOT IN (
                     SELECT user_id FROM monthly_active_users
                     ORDER BY timestamp DESC
                     LIMIT ?
                     )
-                AND user_id NOT IN ({})
-                """.format(','.join(questionmarks))
+                """
+            # Need if/else since 'AND user_id NOT IN ({})' fails on Postgres
+            # when len(reserved_users) == 0. Works fine on sqlite.
+            if len(self.reserved_users) > 0:
+                query_args.extend(self.reserved_users)
+                sql = base_sql + """ AND user_id NOT IN ({})""".format(
+                    ','.join(questionmarks)
+                )
+            else:
+                sql = base_sql
             txn.execute(sql, query_args)
 
         yield self.runInteraction("reap_monthly_active_users", _reap_users)