diff --git a/Lib/abc.py b/Lib/abc.py
index f9b49ac..0f98036 100644
--- a/Lib/abc.py
+++ b/Lib/abc.py
@@ -94,7 +94,7 @@
 
     def register(cls, subclass):
         """Register a virtual subclass of an ABC."""
-        if not isinstance(cls, type):
+        if not isinstance(subclass, type):
             raise TypeError("Can only register classes")
         if issubclass(subclass, cls):
             return  # Already a subclass