UTAH install fails on s390x: gap.size: AttributeError: 'NoneType' object has no attribute 'size'

The UTAH automated (answers.yaml) installs on s390x are failing with this traceback:

 Traceback (most recent call last):
   File "/snap/subiquity/5075/lib/python3.10/site-packages/subiquity/common/api/server.py", line 164, in handler
     result = await implementation(**args)
   File "/snap/subiquity/5075/lib/python3.10/site-packages/subiquity/server/controllers/filesystem.py", line 1017, in v2_guided_GET
     variation.capability_info_for_gap(gap, install_min)
   File "/snap/subiquity/5075/lib/python3.10/site-packages/subiquity/server/controllers/filesystem.py", line 165, in capability_info_for_gap
     if self.capability_info.allowed and gap.size < install_min:
 AttributeError: 'NoneType' object has no attribute 'size'

Full crash report is attached. I couldn't immediately reproduce the issue with an all-defaults manual install on s390x.