diff --git a/bnpm/similarity.py b/bnpm/similarity.py
index 0f30a01..be8b152 100644
--- a/bnpm/similarity.py
+++ b/bnpm/similarity.py
@@ -1,3 +1,4 @@
+from typing import Union, List, Tuple
 import copy
 import time
 from functools import partial