diff --git a/v1/difference_sequence.py b/v1/difference_sequence.py
index c79ecdd..3154385 100644
--- a/v1/difference_sequence.py
+++ b/v1/difference_sequence.py
@@ -102,7 +102,6 @@ def subtract_values(first, second):
 
         index = Index()
         index_out = Index()
-        keys_todo = defaultdict(set)
         output = []
 
         for collection in self._inner: