diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 5aed4dd230..2d60ea6d80 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -146,7 +146,7 @@ class FederationHandler:
             await self.store.get_oldest_event_ids_with_depth_in_room(room_id)
         )
 
-        insertion_events_to_be_backfilled = []
+        insertion_events_to_be_backfilled: Dict[str, int] = {}
         if self.hs.config.experimental.msc2716_enabled:
             insertion_events_to_be_backfilled = (
                 await self.store.get_insertion_event_backwards_extremities_in_room(
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index f4ae4a392c..1b872281f6 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -957,7 +957,9 @@ class EventCreationHandler:
         assert (
             builder.type == EventTypes.Create
             or len(prev_event_ids) > 0
-            or len(auth_event_ids) > 0
+            # Allow an event to have empty list of prev_event_ids
+            # only if it has auth_event_ids.
+            or (auth_event_ids and len(auth_event_ids) > 0)
         ), "Attempting to create an event with no prev_events or auth_event_ids"
 
         event = await builder.build(