diff --git a/Lib/abc.py b/Lib/abc.py
index 54dc8e2..30bb6b0 100644
--- a/Lib/abc.py
+++ b/Lib/abc.py
@@ -137,8 +137,11 @@
         cls._abc_negative_cache_version = ABCMeta._abc_invalidation_counter
         return cls
 
-    def register(cls, subclass):
+    def register(cls, subclass=None):
         """Register a virtual subclass of an ABC."""
+        if subclass is None:
+            raise TypeError("register() cannot be called on an ABCMeta "
+                "subclass, use class Example(metaclass=abc.ABCMeta) instead.")
         if not isinstance(cls, type):
             raise TypeError("Can only register classes")
         if issubclass(subclass, cls):