TypeError: 'NoneType' object is not iterable

Hello,

When I am trying the instructions at:
http://www.virtuatopia.com/index.php/Building_a_Xen_Guest_Root_Filesystem_using_yum_and_rpm

and using yum, I am getting the following errors:
=====================================================
$ yum --installroot=/xen/vm2/root -y groupinstall Base
Setting up Group Process
Setting up repositories
base                      100%
|=========================| 1.1 kB    00:00     
updates                   100%
|=========================|  951 B    00:00     
addons                    100%
|=========================|  951 B    00:00     
extras                    100%
|=========================| 1.1 kB    00:00     
comps.xml                 100%
|=========================| 914 kB    00:03     
Traceback (most recent call last):
  File "/usr/bin/yum", line 27, in <module>
    yummain.main(sys.argv[1:])
  File "/usr/share/yum-cli/yummain.py", line 92, in
main
    result, resultmsgs = do()
  File "/usr/share/yum-cli/cli.py", line 555, in
doCommands
    self.doGroupSetup()
  File
"/var/lib/python-support/python2.5/yum/__init__.py",
line 325, in doGroupSetup
    self.groupInfo.add(groupfile)
  File
"/var/lib/python-support/python2.5/yum/Errors.py",
line 55, in __init__
    YumBaseError.__init__(self)
  File
"/var/lib/python-support/python2.5/yum/Errors.py",
line 25, in __init__
    self.args = args
TypeError: 'NoneType' object is not iterable
=====================================================

Platform: Ubuntu hardy (8.04.1), amd64.

Thanks