diff --git a/synapse/visibility.py b/synapse/visibility.py
index 737328056e3..17ec15f42cb 100644
--- a/synapse/visibility.py
+++ b/synapse/visibility.py
@@ -168,10 +168,11 @@ def allowed(event: EventBase) -> Optional[EventBase]:
         # we won't have such a state. The only outliers that are returned here are the
         # user's own membership event, so we can just inspect that.
 
+        user_membership_event: Optional[EventBase]
         if event.type == EventTypes.Member and event.state_key == user_id:
             user_membership_event = event
         elif state_after_event is not None:
-            user_membership_event = state_after_event[(EventTypes.Member, user_id)]
+            user_membership_event = state_after_event.get((EventTypes.Member, user_id))
         else:
             # unreachable!
             raise Exception("Missing state for event that is not user's own membership")