[CentOS] yum grouplist error

I keep a local copy of the yum repository using rsync every night. Commands like "yum update" still work, but lately I've been getting the following error:

# yum grouplist Setting up Group Process Setting up repositories Error: Caching enabled and local cache: //var/cache/yum/base/ yumgroups.xml does not match checksum

My nightly rsync cron job doesn't update this file -- should it? How does one rebuild it? To set up my local repository, I edited the file /etc/yum.repos.d/CentOS-Base.repo and changed all baseurl entries to use file:///path/to/my/repo instead of http:// mirror.centos/org/... Do I have something misconfigured?

Thanks, Alfred