Why can't I use base_release_0 in #FreeBSD in poudriere? I'm running this:

poudriere jail -c -v 15 -j FreeBSD:15:amd64 -m pkgbase=base_release_0 -U https://pkg.freebsd.org/

pkg: Failed to fetch https://pkg.freebsd.org//FreeBSD:15:amd64/base_release_0/meta.conf: Unknown error

What do I need to read to understand poudriere+pkgbase?