It must be a bug in yum because this happened to me a bunch of times as well. If you run the command again, it'll work (at least it would for me). Are the yum people aware of this?
I filed the bug report a while ago but couldn't reproduce it.
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=123818
reopen the bug and add any strace or any other information you have