Using etcupdate with pkgbase

[Grzegorz Junka]

I have reinstalled the base system from FreeBSD pacakges build on a 
different server. I have also created a tarball to use with etcupdate

root at build:~ # etcupdate build -s /usr/src-12-1-p3 
/usr/obj/usr/src-12-1-p3_etc.tgz

I have extracted it on the destination host

root at host:/tmp # etcupdate extract -t /tmp/src-12-1-p3_etc.tgz

How am I supposed to maintain etc updates going forward? Just running 
etcupdate returns with an error:

root at host:~ # etcupdate
Failed to build new tree.