Can someone explain this weakref behavior?

[Michael Kent]

The Python 2.3.4 docs about weakref say:
Not all objects can be weakly referenced; those objects which can
include class instances, functions written in Python (but not in C),
and methods (both bound and unbound).

I've been unable to get using a bound method as the key in a
WeakKeyDictionary to work.  Using a class instance object works fine
as a key, using a method of that same instance object does not. 
Here's some code, in a file named test_weakref.py:

#! /usr/bin/env python

import unittest
import weakref

class someClass(object):
    def aMethod(self):
        print "Hi!"

class TestCase_01_weakref(unittest.TestCase):

    def test_01_simple(self):

        obj1 = someClass()
        obj2 = someClass()
        wkd = weakref.WeakKeyDictionary()

        wkd[obj1] = 1
        self.assertEqual(len(wkd), 1)

        wkd[obj1.aMethod] = 1
        self.assertEqual(len(wkd), 2)

        wkd[obj2.aMethod] = 1
        self.assertEqual(len(wkd), 3)


if __name__ == "__main__":
    unittest.main()

And here's the output:

./test_weakref.py
F
======================================================================
FAIL: test_01_simple (__main__.TestCase_01_weakref)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "./test_weakref.py", line 22, in test_01_simple
    self.assertEqual(len(wkd), 2)
  File "/usr/local/lib/python2.3/unittest.py", line 302, in
failUnlessEqual
    raise self.failureException, \
AssertionError: 1 != 2

----------------------------------------------------------------------
Ran 1 test in 0.001s

FAILED (failures=1)

It is acting as though a bound method is silently not allowed as the
key in a WeakKeyDictionary.  Can someone set me straight?