Missing return
diff --git a/Lib/_abcoll.py b/Lib/_abcoll.py
index 75a8072..89d4179 100644
--- a/Lib/_abcoll.py
+++ b/Lib/_abcoll.py
@@ -293,6 +293,7 @@
                 self.discard(value)
             else:
                 self.add(value)
+        return self
 
     def __isub__(self, it):
         for value in it: