install: invalid user "mysql" on Gentoo

I just installed Gentoo 2.2 in a Mac-hosted VirtualBox. I seeing the following snippet in /var/log/rc.log:

* Activating additional swap space ...
[ ok ]
* setting up tmpfiles.d entries ...
install: invalid user "mysql"
[ !! ]
* Initializing random number generator ...
[ ok ]

I don't have mysql installed.

What is causing the invalid user message and how do I make it go away?

Solution

I figured out what the problem was. I had mysql set in the USE variable in /etc/portage/make.conf even though I hadn't installed MySQL yet. Doing the following solved the problem:

Remove mysql from USE in /etc/portage/make.conf.
emerge --update --deep --newuse @world
emerge --depclean