WeakValueDictionary retaining reference to object with no more strong references

>>> from weakref import WeakValueDictionary
>>> class Foo(object):
...     pass
>>> foo = Foo()
>>> db = WeakValueDictionary()
>>> db['foo-id'] = foo
>>> del foo
>>> dict(db)
{'foo-id': <__main__.Foo object at 0x4dd946c>}

Why does it show this instead of an empty dictionary? Note that this code produces the result I'd expect:

>>> db2 = WeakValueDictionary()
>>> db2['disposable-id'] = Foo()
>>> dict(db2)
{}

It also behaves as expected when executing a script (instead of the interactive interpreter):

from weakref import WeakValueDictionary
class Foo(object):
    pass
foo = Foo()
db = WeakValueDictionary()
db['foo-id'] = foo
del foo
print str(dict(foo))
# prints {}