1 files changed, 8 insertions, 0 deletions
diff --git a/Lib/abc.py b/Lib/abc.py
index 95126d8a18..681c753215 100644
--- a/Lib/abc.py
+++ b/Lib/abc.py
@@ -4,6 +4,11 @@
 """Abstract Base Classes (ABCs) according to PEP 3119."""
 
 
+# Instance of old-style class
+class _C: pass
+_InstanceType = type(_C())
+
+
 def abstractmethod(funcobj):
     """A decorator indicating abstract methods.
 
@@ -124,6 +129,9 @@ class ABCMeta(type):
         if subclass in cls._abc_cache:
             return True
         subtype = type(instance)
+        # Old-style instances
+        if subtype is _InstanceType:
+            subtype = subclass
         if subtype is subclass or subclass is None:
             if (cls._abc_negative_cache_version ==
                 ABCMeta._abc_invalidation_counter and