diff --git a/u2/list_ops.py b/u2/list_ops.py
index 318f4ea..9bf52c9 100644
--- a/u2/list_ops.py
+++ b/u2/list_ops.py
@@ -9,7 +9,7 @@ def append(list1, list2):
 def concat(lists):
     result = []
     for list in lists:
-        append(result, list)
+        result = append(result, list)
     return result
 
 def filter(function, list):