diff --git a/src/stores/TagOrderStore.js b/src/stores/TagOrderStore.js
index 8ecc47d430..d26394b0d4 100644
--- a/src/stores/TagOrderStore.js
+++ b/src/stores/TagOrderStore.js
@@ -180,7 +180,10 @@ class TagOrderStore extends Store {
             const client = MatrixClientPeg.get();
             const changedBadges = {};
             groupIds.forEach(groupId => {
-                const rooms = GroupStore.getGroupRooms(groupId).map(r => client.getRoom(r.roomId));
+                const rooms =
+                    GroupStore.getGroupRooms(groupId)
+                    .map(r => client.getRoom(r.roomId)) // to Room objects
+                    .filter(r => r !== null && r !== undefined);   // filter out rooms we haven't joined from the group
                 const badge = rooms && RoomNotifs.aggregateNotificationCount(rooms);
                 changedBadges[groupId] = (badge && badge.count !== 0) ? badge : undefined;
             });