diff --git a/synapse/replication/slave/storage/filtering.py b/synapse/replication/slave/storage/filtering.py
index 5037f395b9..819ed62881 100644
--- a/synapse/replication/slave/storage/filtering.py
+++ b/synapse/replication/slave/storage/filtering.py
@@ -21,4 +21,5 @@ class SlavedFilteringStore(BaseSlavedStore):
     def __init__(self, db_conn, hs):
         super(SlavedFilteringStore, self).__init__(db_conn, hs)
 
+    # Filters are immutable so this cache doesn't need to be expired
     get_user_filter = FilteringStore.__dict__["get_user_filter"]