diff --git a/synapse/storage/state.py b/synapse/storage/state.py
index 0bea7374f7..24503cd5aa 100644
--- a/synapse/storage/state.py
+++ b/synapse/storage/state.py
@@ -31,6 +31,9 @@ MAX_STATE_DELTA_HOPS = 100
 
 
 class _GetStateGroupDelta(namedtuple("_GetStateGroupDelta", ("prev_group", "delta_ids"))):
+    """Return type of get_state_group_delta that implements __len__, which lets
+    us use the itrable flag when caching
+    """
     __slots__ = []
 
     def __len__(self):