AutoClass and Instance Attributes

According to the sphinx documentation, the directive .. autoattributeshould be able to document instance attributes. However, if I do ::

.. currentmodule:: xml.etree.ElementTree

.. autoclass:: ElementTree

   .. autoattribute:: ElementTree._root

Then, when I create it, I get the AttributeError attribute:

Traceback (most recent call last):etree.ElementTree.ElementTree                 
  File "/Volumes/Raptor/Library/Python/2.7/lib/python/site-packages/sphinx/ext/autodoc.py", line 326, in import_object
    obj = self.get_attr(obj, part)
  File "/Volumes/Raptor/Library/Python/2.7/lib/python/site-packages/sphinx/ext/autodoc.py", line 232, in get_attr
    return safe_getattr(obj, name, *defargs)
  File "/Volumes/Raptor/Library/Python/2.7/lib/python/site-packages/sphinx/util/inspect.py", line 70, in safe_getattr
    raise AttributeError(name)
AttributeError: _root

although if I create an instance ElementTreeand try to access the attribute _root, it works fine:

>>> from xml.etree.ElementTree import ElementTree
>>> e = ElementTree()
>>> hasattr(e, '_root')
True

What am I doing wrong?

(I have this problem with one of my own classes, but I just use the ElementTree class as an example, as it is in the standard library)