diff --git a/synapse/federation/sender/__init__.py b/synapse/federation/sender/__init__.py
index 8bb17b3a05..e33b29a42c 100644
--- a/synapse/federation/sender/__init__.py
+++ b/synapse/federation/sender/__init__.py
@@ -297,6 +297,8 @@ class FederationSender:
         sent_pdus_destination_dist_total.inc(len(destinations))
         sent_pdus_destination_dist_count.inc()
 
+        assert pdu.internal_metadata.stream_ordering
+
         # track the fact that we have a PDU for these destinations,
         # to allow us to perform catch-up later on if the remote is unreachable
         # for a while.
diff --git a/synapse/federation/sender/per_destination_queue.py b/synapse/federation/sender/per_destination_queue.py
index bc99af3fdd..db8e456fe8 100644
--- a/synapse/federation/sender/per_destination_queue.py
+++ b/synapse/federation/sender/per_destination_queue.py
@@ -158,6 +158,7 @@ class PerDestinationQueue:
             # yet know if we have anything to catch up (None)
             self._pending_pdus.append(pdu)
         else:
+            assert pdu.internal_metadata.stream_ordering
             self._catchup_last_skipped = pdu.internal_metadata.stream_ordering
 
         self.attempt_new_transaction()
@@ -361,6 +362,7 @@ class PerDestinationQueue:
                         last_successful_stream_ordering = (
                             final_pdu.internal_metadata.stream_ordering
                         )
+                        assert last_successful_stream_ordering
                         await self._store.set_destination_last_successful_stream_ordering(
                             self._destination, last_successful_stream_ordering
                         )
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 1a8144405a..5ac2fc5656 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -3008,6 +3008,9 @@ class FederationHandler(BaseHandler):
         elif event.internal_metadata.is_outlier():
             return
 
+        # the event has been persisted so it should have a stream ordering.
+        assert event.internal_metadata.stream_ordering
+
         event_pos = PersistedEventPosition(
             self._instance_name, event.internal_metadata.stream_ordering
         )
diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 78e645592f..b4abd961b9 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -331,6 +331,10 @@ class PersistEventsStore:
         min_stream_order = events_and_contexts[0][0].internal_metadata.stream_ordering
         max_stream_order = events_and_contexts[-1][0].internal_metadata.stream_ordering
 
+        # stream orderings should have been assigned by now
+        assert min_stream_order
+        assert max_stream_order
+
         self._update_forward_extremities_txn(
             txn,
             new_forward_extremities=new_forward_extremeties,
diff --git a/synapse/storage/persist_events.py b/synapse/storage/persist_events.py
index 72939f3984..4d2d88d1f0 100644
--- a/synapse/storage/persist_events.py
+++ b/synapse/storage/persist_events.py
@@ -248,6 +248,8 @@ class EventsPersistenceStorage:
         await make_deferred_yieldable(deferred)
 
         event_stream_id = event.internal_metadata.stream_ordering
+        # stream ordering should have been assigned by now
+        assert event_stream_id
 
         pos = PersistedEventPosition(self._instance_name, event_stream_id)
         return pos, self.main_store.get_room_max_token()