FreeBSD poudriere stable@339445/MANIFEST: Not Found

# date ; uname -v
Sat 20 Oct 2018 14:53:59 BST
FreeBSD 12.0-BETA1 r339438 GENERIC
# poudriere jail -c -v stable@339445 -j stable
[00:00:00] Creating stable fs at /usr/local/poudriere/jails/stable... done
[00:00:01] FREEBSD_HOST from config invalid; defaulting to https://download.FreeBSD.org
[00:00:01] Fetching MANIFEST for FreeBSD stable@339445 amd64
fetch: https://download.FreeBSD.org/ftp/releases/amd64/amd64/stable@339445/MANIFEST: Not Found
fetch: https://download.FreeBSD.org/ftp/releases/amd64/amd64/stable@339445/MANIFEST: Not Found
[00:00:01] Error: Failed to fetch from https://download.FreeBSD.org/ftp/releases/amd64/amd64/stable@339445/MANIFEST
[00:00:01] Error while creating jail, cleaning up.
[00:00:01] Removing stable jail... done
[00:00:02] Cleaning stable data... done
#