diff -r c3a09c535001 Lib/importlib/_bootstrap.py
--- a/Lib/importlib/_bootstrap.py	Sat Feb 23 08:40:39 2013 +0200
+++ b/Lib/importlib/_bootstrap.py	Sat Feb 23 17:16:32 2013 +0100
@@ -510,7 +510,7 @@
     """Set __loader__ on the returned module."""
     def set_loader_wrapper(self, *args, **kwargs):
         module = fxn(self, *args, **kwargs)
-        if not hasattr(module, '__loader__'):
+        if getattr(module, '__loader__', None) is None:
             module.__loader__ = self
         return module
     _wrap(set_loader_wrapper, fxn)
@@ -1567,7 +1567,7 @@
         except AttributeError:
             pass
     # Set loader if need be.
-    if not hasattr(module, '__loader__'):
+    if getattr(module, '__loader__', None) is None:
         try:
             module.__loader__ = loader
         except AttributeError:
@@ -1728,7 +1728,7 @@
     module_type = type(sys)
     for name, module in sys.modules.items():
         if isinstance(module, module_type):
-            if not hasattr(module, '__loader__'):
+            if getattr(module, '__loader__', None) is None:
                 if name in sys.builtin_module_names:
                     module.__loader__ = BuiltinImporter
                 elif _imp.is_frozen(name):