FC3 daily yum.cron reports repomd.xml broken, how to repair and prevent?

[Rhesa Rozendaal]

Got this report in the mail:

/etc/cron.daily/yum.cron:

//var/cache/yum/base/repomd.xml:1: parser error : Document is empty

^
//var/cache/yum/base/repomd.xml:1: parser error : Start tag expected, '<' not 
found

^
Cannot open/read repomd.xml file for repository: base
Error importing repomd.xml from base: Error: could not parse file 
//var/cache/yum/base/repomd.xml


How the hell did that happen, and what should I do to prevent it from 
happening again, and what should I do to repair it?

TIA,
Rhesa