YumRepo Error: All mirror URLs are not using ftp, http[s] or file.

Hello.  I'm getting this error when running yum with --installroot (it
works fine without).  I've tried copying all [i think] of the yum
config files from the /etc to etc under the installroot.  Googling,
including the archives of this list, didn't help.  This is on CentOS
5.5.    Any ideas or tips?

Thanks...
-- Chad

$ sudo yum -y --installroot=/mnt/stage2 groupinstall Base
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
 Eg. $releasever is not a valid release or hasnt been released yet/
removing mirrorlist with no valid mirrors:
/mnt/stage2/var/cache/yum/addons/mirrorlist.txt
Error: Cannot find a valid baseurl for repo: addons