diff --git a/synapse/federation/federation_client.py b/synapse/federation/federation_client.py
index a163b2674..4b3bf9783 100644
--- a/synapse/federation/federation_client.py
+++ b/synapse/federation/federation_client.py
@@ -523,7 +523,7 @@ class FederationClient(FederationBase):
             # Are we missing any?
 
             seen_events = set(earliest_events_ids)
-            seen_events.update(e.event_id for e in signed_events)
+            seen_events.update(e.event_id for e in signed_events if e)
 
             missing_events = {}
             for e in itertools.chain(latest_events, signed_events):