chroot backend failing to build multiple image-targets

{"project": "ubuntu-cpc", "image_targets": ["tarball", "squashfs"], "image_format": "ext4"}

in livefs build fails for me with:

make-hooks: Series file for image set ''tarball' not found.
Build failed
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/lpbuildd/target/build_livefs.py", line 176, in run
    self.build()
  File "/usr/lib/python2.7/dist-packages/lpbuildd/target/build_livefs.py", line 166, in build
    self.run_build_command(["lb", "config"], env=lb_env)
  File "/usr/lib/python2.7/dist-packages/lpbuildd/target/build_livefs.py", line 90, in run_build_command
    return self.backend.run(args, cwd="/build", **kwargs)
  File "/usr/lib/python2.7/dist-packages/lpbuildd/target/chroot.py", line 88, in run
    subprocess.check_call(cmd, **kwargs)
  File "/usr/lib/python2.7/subprocess.py", line 190, in check_call
    raise CalledProcessError(retcode, cmd)
CalledProcessError: Command '['sudo', '/usr/sbin/chroot', '/home/buildd/build-LIVEFSBUILD-223983/chroot-autobuild', '/bin/sh', '-c', 'cd /build && linux64 env PROJECT=ubuntu-cpc ARCH=riscv64 \'IMAGE_TARGETS=\'"\'"\'tarball squashfs\'"\'"\'\' SUITE=focal NOW=20200624-112500 IMAGEFORMAT=ext4 lb config']' returned non-zero exit status 1

https://launchpadlibrarian.net/485684996/buildlog_ubuntu_focal_riscv64_any_BUILDING.txt.gz

Now the environment passed to lb build, looks very odd. I.e. \'IMAGE_TARGETS=\'"\'"\'tarball squashfs\'"\'"\'\' is like over encoded maybe?