How to get en_US.UTF-8 locale in guix shell

[Nathan Dehnel]

After following these instructions
https://guix.gnu.org/manual/en/html_node/Application-Setup.html#locales_002dand_002dlocpath
locale -a only returns
C
POSIX
as the locales inside a guix shell. What else do I need to do?
glibc-locales is installed both inside and outside the shell.