diff --git a/Lib/_abcoll.py b/Lib/_abcoll.py
index 7b01178a054..a60d91e7cb8 100644
--- a/Lib/_abcoll.py
+++ b/Lib/_abcoll.py
@@ -598,6 +598,7 @@ class MutableSequence(Sequence):
 
     def __iadd__(self, values):
         self.extend(values)
+        return self
 
 MutableSequence.register(list)
 MutableSequence.register(bytearray)  # Multiply inheriting, see ByteString