diff --git a/changelog.d/4081.bugfix b/changelog.d/4081.bugfix
index 13dad5884..cfe4b3e9d 100644
--- a/changelog.d/4081.bugfix
+++ b/changelog.d/4081.bugfix
@@ -1 +1,2 @@
-Fix race condition in populating reserved users
+Fix race condition where config defined reserved users were not being added to
+the monthly active user list prior to the homeserver reactor firing up
diff --git a/synapse/storage/monthly_active_users.py b/synapse/storage/monthly_active_users.py
index cf15f8c5b..9a5c3b7ed 100644
--- a/synapse/storage/monthly_active_users.py
+++ b/synapse/storage/monthly_active_users.py
@@ -33,21 +33,23 @@ class MonthlyActiveUsersStore(SQLBaseStore):
         self._clock = hs.get_clock()
         self.hs = hs
         self.reserved_users = ()
-        self.initialise_reserved_users(
-            dbconn.cursor(), hs.config.mau_limits_reserved_threepids
+        # Do not add more reserved users than the total allowable number
+        self._initialise_reserved_users(
+            dbconn.cursor(),
+            hs.config.mau_limits_reserved_threepids[:self.hs.config.max_mau_value],
         )
 
-    def initialise_reserved_users(self, txn, threepids):
+    def _initialise_reserved_users(self, txn, threepids):
         """Ensures that reserved threepids are accounted for in the MAU table, should
         be called on start up.
 
-        Arguments:
-            threepids []: List of threepid dicts to reserve
+        Args:
+            txn (cursor):
+            threepids (list[dict]): List of threepid dicts to reserve
         """
         reserved_user_list = []
 
-        # Do not add more reserved users than the total allowable number
-        for tp in threepids[:self.hs.config.max_mau_value]:
+        for tp in threepids:
             user_id = self.get_user_id_by_threepid_txn(
                 txn,
                 tp["medium"], tp["address"]
@@ -172,26 +174,36 @@ class MonthlyActiveUsersStore(SQLBaseStore):
 
     @defer.inlineCallbacks
     def upsert_monthly_active_user(self, user_id):
-        """Updates or inserts monthly active user member
-        Arguments:
+        """Updates or inserts the user into the monthly active user table, which
+        is used to track the current MAU usage of the server
+
+        Args:
             user_id (str): user to add/update
         """
         is_insert = yield self.runInteraction(
             "upsert_monthly_active_user", self.upsert_monthly_active_user_txn,
             user_id
         )
+        # Considered pushing cache invalidation down into txn method, but
+        # did not because txn is not a LoggingTransaction. This means I could not
+        # call txn.call_after(). Therefore cache is altered in background thread
+        # and calls from elsewhere to user_last_seen_monthly_active and
+        # get_monthly_active_count fail with ValueError in
+        #  synapse/util/caches/descriptors.py#check_thread
         if is_insert:
             self.user_last_seen_monthly_active.invalidate((user_id,))
             self.get_monthly_active_count.invalidate(())
 
     def upsert_monthly_active_user_txn(self, txn, user_id):
-        """
-            Updates or inserts monthly active user member
-            Arguments:
-                txn (cursor):
-                user_id (str): user to add/update
+        """Updates or inserts monthly active user member
+
+        Args:
+            txn (cursor):
+            user_id (str): user to add/update
+
+        Returns:
             bool: True if a new entry was created, False if an
-                existing one was updated.
+            existing one was updated.
         """
         # Am consciously deciding to lock the table on the basis that is ought
         # never be a big table and alternative approaches (batching multiple
@@ -207,6 +219,7 @@ class MonthlyActiveUsersStore(SQLBaseStore):
                 "timestamp": int(self._clock.time_msec()),
             },
         )
+
         return is_insert
 
     @cached(num_args=1)
diff --git a/synapse/storage/registration.py b/synapse/storage/registration.py
index 0f970850e..80d76bf9d 100644
--- a/synapse/storage/registration.py
+++ b/synapse/storage/registration.py
@@ -474,6 +474,15 @@ class RegistrationStore(RegistrationWorkerStore,
 
     @defer.inlineCallbacks
     def get_user_id_by_threepid(self, medium, address):
+        """Returns user id from threepid
+
+        Args:
+            medium (str): threepid medium e.g. email
+            address (str): threepid address e.g. me@example.com
+
+        Returns:
+            Deferred[str|None]: user id or None if no user id/threepid mapping exists
+        """
         user_id = yield self.runInteraction(
             "get_user_id_by_threepid", self.get_user_id_by_threepid_txn,
             medium, address
@@ -481,6 +490,16 @@ class RegistrationStore(RegistrationWorkerStore,
         defer.returnValue(user_id)
 
     def get_user_id_by_threepid_txn(self, txn, medium, address):
+        """Returns user id from threepid
+
+        Args:
+            txn (cursor):
+            medium (str): threepid medium e.g. email
+            address (str): threepid address e.g. me@example.com
+
+        Returns:
+            str|None: user id or None if no user id/threepid mapping exists
+        """
         ret = self._simple_select_one_txn(
             txn,
             "user_threepids",
diff --git a/tests/storage/test_monthly_active_users.py b/tests/storage/test_monthly_active_users.py
index 0c17745ae..832e379a8 100644
--- a/tests/storage/test_monthly_active_users.py
+++ b/tests/storage/test_monthly_active_users.py
@@ -54,7 +54,7 @@ class MonthlyActiveUsersTestCase(HomeserverTestCase):
         self.store.user_add_threepid(user2, "email", user2_email, now, now)
 
         self.store.runInteraction(
-            "initialise", self.store.initialise_reserved_users, threepids
+            "initialise", self.store._initialise_reserved_users, threepids
         )
         self.pump()
 
@@ -203,7 +203,7 @@ class MonthlyActiveUsersTestCase(HomeserverTestCase):
         ]
         self.hs.config.mau_limits_reserved_threepids = threepids
         self.store.runInteraction(
-            "initialise", self.store.initialise_reserved_users, threepids
+            "initialise", self.store._initialise_reserved_users, threepids
         )
 
         self.pump()