diff --git a/synapse/event_auth.py b/synapse/event_auth.py
index 40e508f5b4a..52a78f51aa9 100644
--- a/synapse/event_auth.py
+++ b/synapse/event_auth.py
@@ -177,7 +177,7 @@ async def check_state_independent_auth_rules(
     # 2. Reject if event has auth_events that: ...
     auth_events: MutableMapping[str, "EventBase"]
     if batched_auth_events:
-        auth_events = batched_auth_events
+        auth_events = dict(batched_auth_events)
         needed_auth_event_ids = [
             event_id
             for event_id in event.auth_event_ids()