diff --git a/u6/matching_brackets.py b/u6/matching_brackets.py
index 2ab7368..16ffc8f 100644
--- a/u6/matching_brackets.py
+++ b/u6/matching_brackets.py
@@ -1,5 +1,6 @@
 def is_paired (input_string):
     pairs = {")": "(", "]": "[", "}": "{"}
+    opening = set(pairs.values())
     stack = []
     for char in input_string:
         if char in opening: