diff --git a/synapse/federation/federation_base.py b/synapse/federation/federation_base.py
index e8f3bff2a40a..190ea1fba1f2 100644
--- a/synapse/federation/federation_base.py
+++ b/synapse/federation/federation_base.py
@@ -104,8 +104,10 @@ def handle_check_result(pdu: EventBase, deferred: Deferred):
 
             if not res and pdu.origin != origin:
                 try:
+                    # This should not exist in the base implementation, until
+                    # this is fixed, ignore it for typing. See issue #6997.
                     res = yield defer.ensureDeferred(
-                        self.get_pdu(
+                        self.get_pdu(  # type: ignore
                             destinations=[pdu.origin],
                             event_id=pdu.event_id,
                             room_version=room_version,