zope.testrunner 4.0.0b2 uninstallable when using Python 2.4

This should be self-explanatory:

$ virtualenv -p python2.4 /tmp/sandbox --no-site-packages
$ /tmp/sandbox/bin/easy_install zope.testrunner
Searching for zope.testrunner
Reading http://pypi.python.org/simple/zope.testrunner/
Best match: zope.testrunner 4.0.0b2
Downloading http://pypi.python.org/packages/source/z/zope.testrunner/zope.testrunner-4.0.0b2.tar.gz#md5=e946ea5c3934590229acddb76d859419
Processing zope.testrunner-4.0.0b2.tar.gz
Running zope.testrunner-4.0.0b2/setup.py -q bdist_egg --dist-dir /tmp/easy_install-AFiv7E/zope.testrunner-4.0.0b2/egg-dist-tmp--kb-lZ
package init file 'src/zope/testrunner/testrunner-ex/__init__.py' not found (or not a regular file)
error: Setup script exited with error: can't copy 'src/zope/testrunner/testrunner-ex-251759/eggs/foo.bar-1.2-py2.5.egg/foo/bar/__init__.py': doesn't exist or not a regular file

(It works if I use python2.5 instead of 2.4.)