diff --git a/crdt/utils.py b/crdt/utils.py
index fa4f0db..47d0655 100644
--- a/crdt/utils.py
+++ b/crdt/utils.py
@@ -1,5 +1,7 @@
 # -*- coding: utf-8 -*-
 import bisect
+from collections import MutableSet
+
 
 class SortedSet(MutableSet):
     def __init__(self, items):