1 files changed, 1 insertions, 1 deletions
diff --git a/synapse/state/__init__.py b/synapse/state/__init__.py
index 31082bb16a..5b0900aa3c 100644
--- a/synapse/state/__init__.py
+++ b/synapse/state/__init__.py
@@ -738,7 +738,7 @@ def _make_state_cache_entry(
 
     # failing that, look for the closest match.
     prev_group = None
-    delta_ids = None
+    delta_ids = None  # type: Optional[StateMap[str]]
 
     for old_group, old_state in state_groups_ids.items():
         n_delta_ids = {k: v for k, v in new_state.items() if old_state.get(k) != v}