diff --git a/synapse/api/filtering.py b/synapse/api/filtering.py
index f3a056110f..3906475403 100644
--- a/synapse/api/filtering.py
+++ b/synapse/api/filtering.py
@@ -448,7 +448,7 @@ class Filter(object):
         """Returns a new filter with the given room IDs appended.
 
         Args:
-            room_ids (list): A list of room_ids.
+            room_ids (iterable[unicode]): The room_ids to add
 
         Returns:
             filter: A new filter including the given rooms and the old