Introduction of subplatform around 18.5 broke ConfigDrive

I create custom LXD images from Ubuntu minimal images for an OpenStack environment based on LXD containers and the images I created worked on Ubuntu 18.04 hosts until around the cloud-init v18.5 release where subplatform was introduced. My images produced this error:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/cloudinit/sources/__init__.py", line 760, in find_source
    if s.update_metadata([EventType.BOOT_NEW_INSTANCE]):
  File "/usr/lib/python3/dist-packages/cloudinit/sources/__init__.py", line 649, in update_metadata
    result = self.get_data()
  File "/usr/lib/python3/dist-packages/cloudinit/sources/__init__.py", line 276, in get_data
    self.persist_instance_data()
  File "/usr/lib/python3/dist-packages/cloudinit/sources/__init__.py", line 306, in persist_instance_data
    self._get_standardized_metadata())
  File "/usr/lib/python3/dist-packages/cloudinit/sources/__init__.py", line 246, in _get_standardized_metadata
    'subplatform': self.subplatform}}
  File "/usr/lib/python3/dist-packages/cloudinit/sources/__init__.py", line 419, in subplatform
    self._subplatform = self._get_subplatform()
  File "/usr/lib/python3/dist-packages/cloudinit/sources/DataSourceConfigDrive.py", line 170, in _get_subplatform
    return '%s (%s)' % (subplatform_type, self.source)

This code uses and if/elif which leaves subplatform_type undefined on my image. I don't have any idea what this is supposed to be, but looking at other DataSources, they use an if/else so that subplatform_type is alway defined. The diff that makes this work for me and works like other DataSources is attached.