diff -r 4c1b0c4405f6 Lib/_weakrefset.py
--- a/Lib/_weakrefset.py	Mon Nov 08 13:57:59 2010 +0100
+++ b/Lib/_weakrefset.py	Mon Nov 08 11:57:40 2010 -0500
@@ -66,7 +66,12 @@
         return sum(x() is not None for x in self.data)
 
     def __contains__(self, item):
-        return ref(item) in self.data
+        try:
+            weak = ref(item)
+        except TypeError:
+            return False
+        else:
+            return weak in self.data
 
     def __reduce__(self):
         return (self.__class__, (list(self),),
diff -r 4c1b0c4405f6 Lib/test/test_weakset.py
--- a/Lib/test/test_weakset.py	Mon Nov 08 13:57:59 2010 +0100
+++ b/Lib/test/test_weakset.py	Mon Nov 08 11:57:40 2010 -0500
@@ -62,7 +62,14 @@
     def test_contains(self):
         for c in self.letters:
             self.assertEqual(c in self.s, c in self.d)
-        self.assertRaises(TypeError, self.s.__contains__, [[]])
+        # Don't raise for un-weakref-able types
+        self.assertNotIn(None, self.s)
+        self.assertNotIn([[]], self.s)
+        class NoHash(object):
+            def __hash__(self):
+                raise TypeError('Not hashable')
+        # ...but permit any hashing / comparison exceptions to propagate.
+        self.assertRaises(TypeError, self.s.__contains__, NoHash())
         self.assertIn(self.obj, self.fs)
         del self.obj
         self.assertNotIn(SomeClass('F'), self.fs)