diff --git a/synapse/handlers/device.py b/synapse/handlers/device.py
index 815410969c..af28e1e009 100644
--- a/synapse/handlers/device.py
+++ b/synapse/handlers/device.py
@@ -266,13 +266,14 @@ class DeviceHandler(BaseHandler):
                     if not prev_event_id or prev_event_id != event_id:
                         possibly_changed.add(state_key)
 
-        user_ids_changed = set()
-        for other_user_id in possibly_changed:
-            other_rooms = yield self.store.get_rooms_for_user(other_user_id)
-            if room_ids.intersection(e.room_id for e in other_rooms):
-                user_ids_changed.add(other_user_id)
+        users_who_share_room = yield self.store.get_users_who_share_room_with_user(
+            user_id
+        )
 
-        defer.returnValue(user_ids_changed)
+        # We return the intersection of users whose devices have changed (or
+        # membership has changeD) and the users who share a room with the
+        # requester
+        defer.returnValue(users_who_share_room & users_who_share_room)
 
     @defer.inlineCallbacks
     def _incoming_device_list_update(self, origin, edu_content):
diff --git a/synapse/rest/client/v2_alpha/keys.py b/synapse/rest/client/v2_alpha/keys.py
index f99b53530a..e3f3ea86b7 100644
--- a/synapse/rest/client/v2_alpha/keys.py
+++ b/synapse/rest/client/v2_alpha/keys.py
@@ -193,7 +193,7 @@ class KeyChangesServlet(RestServlet):
         )
 
         defer.returnValue((200, {
-            "changed": changed
+            "changed": list(changed)
         }))