py:choose not working

With the reference implementation of genshi you can do the following:
>>> from genshi.template import MarkupTemplate
>>> tmp = MarkupTemplate("""\
... <div xmlns="http://www.w3.org/1999/xhtml"
... xmlns:py="http://genshi.edgewall.org/">
... <py:choose test="">
... <span py:when="0 == 1">0</span>
... <span py:when="1 == 1">1</span>
... </py:choose>
... </div>""")
>>> stream = tmp.generate()
>>> print stream.render('xhtml')
<div xmlns="http://www.w3.org/1999/xhtml">
    <span>1</span>
</div>

The same doesn't work in chameleon.genshi.