Fails with BINPKG_COMPRESS=xz

[HJarausch]

I have BINPKG_COMPRESS=xz in make.conf.
Here quickunpkg fails with
17 # quickunpkg portage-9999.tbz2

Create the workdir, tbz2file

TBZ2FILE=portage-9999.tbz2
TBZ2FILE_XPAK=portage-9999.xpak
WORKDIR=/tmp/20180510_091345
mkdir: created directory '/tmp/20180510_091345'

Copying the file to the workdir

'portage-9999.tbz2' -> '/tmp/20180510_091345/portage-9999.tbz2'

Uncompressing the tbz2file

input tbz2: portage-9999.tbz2 (3.3M)
output tar.bz2: (null) (3.3M)
output xpak: portage-9999.xpak (43.0K)
PF=portage-9999
CATEGORY=sys-apps
TBZ2FILE_XPAK=portage-9999.xpak

Emerging portage-9999.tbz2

mkdir: created directory '/tmp/20180510_091345/All'
'portage-9999.tbz2' -> '/tmp/20180510_091345/All/portage-9999.tbz2'

!!! Invalid binary package: '/tmp/20180510_091345/portage-9999.xpak'
!!! Missing metadata key(s): CATEGORY, PF, SLOT. This binary package is not
!!! recoverable and should be deleted.

!!! Invalid binary package: '/tmp/20180510_091345/portage-9999.xpak'
!!! Missing metadata key(s): CATEGORY, PF, SLOT. This binary package is not
!!! recoverable and should be deleted.