$ ubuntu-image -c edge -O xx ../pc.model 
Fetching core
Fetching pc-kernel
Fetching pc
COMMAND FAILED: mkfs.ext4 -L writable -T default -O uninit_bg /tmp/tmpmarcuw6a/volumes/pc/part3.img

mke2fs 1.42.13 (17-May-2015)
Se puso una opción no válida para el sistema de archivos: has_journal,extent,huge_file,flex_bg,metadata_csum,64bit,dir_nlink,extra_isize

Crash in state machine
Traceback (most recent call last):
  File "/snap/ubuntu-image/64/lib/python3/site-packages/ubuntu_image/__main__.py", line 204, in main
    list(state_machine)
  File "/snap/ubuntu-image/64/lib/python3/site-packages/ubuntu_image/state.py", line 82, in __next__
    step()
  File "/snap/ubuntu-image/64/lib/python3/site-packages/ubuntu_image/builder.py", line 415, in populate_filesystems
    self._populate_one_volume(name, volume)
  File "/snap/ubuntu-image/64/lib/python3/site-packages/ubuntu_image/builder.py", line 368, in _populate_one_volume
    mkfs_ext4(part_img, self.rootfs, part.filesystem_label)
  File "/snap/ubuntu-image/64/lib/python3/site-packages/ubuntu_image/helpers.py", line 154, in mkfs_ext4
    run('mkfs.ext4 -L {} -T default -O uninit_bg {}'.format(label, img_file))
  File "/snap/ubuntu-image/64/lib/python3/site-packages/ubuntu_image/helpers.py", line 98, in run
    proc.check_returncode()
  File "/snap/ubuntu-image/64/usr/lib/python3.5/subprocess.py", line 659, in check_returncode
    self.stderr)
subprocess.CalledProcessError: Command '['mkfs.ext4', '-L', 'writable', '-T', 'default', '-O', 'uninit_bg', '/tmp/tmpmarcuw6a/volumes/pc/part3.img']' returned non-zero exit status 1