diff --git a/Lib/_abcoll.py b/Lib/_abcoll.py
index 990ff00..692a0d7 100644
--- a/Lib/_abcoll.py
+++ b/Lib/_abcoll.py
@@ -560,5 +560,6 @@
 
     def __iadd__(self, values):
         self.extend(values)
+        return self
 
 MutableSequence.register(list)