diff --git a/synapse/storage/databases/main/end_to_end_keys.py b/synapse/storage/databases/main/end_to_end_keys.py
index 38d8785faa..d36df75bb8 100644
--- a/synapse/storage/databases/main/end_to_end_keys.py
+++ b/synapse/storage/databases/main/end_to_end_keys.py
@@ -240,7 +240,7 @@ class EndToEndKeyWorkerStore(EndToEndKeyBackgroundStore, CacheInvalidationWorker
                 if r is None:
                     continue
 
-                r["unsigned"] = {}
+                r.setdefault("unsigned", {})
                 if include_displaynames:
                     # Include the device's display name in the "unsigned" dictionary
                     display_name = device_info.display_name