diff --git a/Lib/test/test_descr.py b/Lib/test/test_descr.py
index b108395..8927907 100644
--- a/Lib/test/test_descr.py
+++ b/Lib/test/test_descr.py
@@ -3623,6 +3623,13 @@
         raise TestFailed, "shouldn't be able to assign to list.__bases__"
 
     try:
+        D.__bases__ = (C2, list)
+    except TypeError:
+        pass
+    else:
+        assert 0, "best_base calculation found wanting"
+
+    try:
         del D.__bases__
     except TypeError:
         pass