diff --git a/synapse/util/caches/treecache.py b/synapse/util/caches/treecache.py
index 6faf2cf75..2d014e1f9 100644
--- a/synapse/util/caches/treecache.py
+++ b/synapse/util/caches/treecache.py
@@ -2,6 +2,11 @@ SENTINEL = object()
 
 
 class TreeCache(object):
+    """
+    Tree-based backing store for LruCache. Allows subtrees of data to be deleted
+    efficiently.
+    Keys must be tuples.
+    """
     def __init__(self):
         self.root = {}