Cannot make an Ubuntu Core18 image with generic model assertion

When attempting to create a snappy ubuntu core18 image with the generic Ubuntu core18 amd64 model assertion file, it fails to find the gadget snap “pc=18” (as well as the kernel snap if I use a custom snap).

Here is the full debug log:

$ sudo ubuntu-image snap --debug ubuntu-core-18-amd64.model-assertion
DEBUG:ubuntu-image:-> [ 0] make_temporary_directories
DEBUG:ubuntu-image:-> [ 1] prepare_gadget_tree
DEBUG:ubuntu-image:-> [ 2] prepare_image
error: cannot download snap "pc=18": snap not found
ERROR:ubuntu-image:COMMAND FAILED: snap prepare-image ubuntu-core-18-amd64.model-assertion /tmp/tmpwez8h8o6/unpack
ERROR:ubuntu-image:Full debug traceback follows
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/ubuntu_image/", line 22, in prepare_image, self.args.extra_snaps)
  File "/usr/lib/python3/dist-packages/ubuntu_image/", line 141, in snap
    run(cmd, stdout=None, stderr=None, env=os.environ)
  File "/usr/lib/python3/dist-packages/ubuntu_image/", line 122, in run
  File "/usr/lib/python3.5/", line 659, in check_returncode
subprocess.CalledProcessError: Command '['snap', 'prepare-image', 'ubuntu-core-18-amd64.model-assertion', '/tmp/tmpwez8h8o6/unpack']' returned non-zero exit status 1

Has anyone had any luck running this ubuntu-image command? I am using Ubuntu Server 16.04LTS for a build environment