1 files changed, 3 insertions, 1 deletions
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index b1784638f4..32451670f3 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -1123,7 +1123,9 @@ class RoomCreationHandler:
                 event_dict,
                 prev_event_ids=prev_event,
                 depth=depth,
-                state_map=state_map,
+                # Take a copy to ensure each event gets a unique copy of
+                # state_map since it is modified below.
+                state_map=dict(state_map),
                 for_batch=for_batch,
             )