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
diff --git a/Lib/test/test_abc.py b/Lib/test/test_abc.py
index c9deb4b..a6e7062 100644
--- a/Lib/test/test_abc.py
+++ b/Lib/test/test_abc.py
@@ -139,6 +139,12 @@
         self.assertRaises(RuntimeError, C.register, A)  # cycles not allowed
         C.register(B)  # ok
 
+    def test_register_non_class(self):
+        class A(metaclass=abc.ABCMeta):
+            pass
+        self.assertRaisesRegexp(TypeError, "Can only register classes",
+                                A.register, 4)
+
     def test_registration_transitiveness(self):
         class A(metaclass=abc.ABCMeta):
             pass