$ git ubuntu merge finish ubuntu/devel 
Traceback (most recent call last):
  File "/snap/git-ubuntu/122/bin/git-ubuntu", line 18, in <module>
    main()
  File "/snap/git-ubuntu/122/gitubuntu/__main__.py", line 213, in main
    args.func(args)
  File "/snap/git-ubuntu/122/gitubuntu/merge.py", line 406, in main
    self.do_finish()
  File "/snap/git-ubuntu/122/gitubuntu/merge.py", line 308, in do_finish
    self.do_merge_changelogs()
  File "/snap/git-ubuntu/122/gitubuntu/merge.py", line 179, in do_merge_changelogs
    self.commitish, 'debian/changelog')
  File "/snap/git-ubuntu/122/gitubuntu/git_repository.py", line 719, in extract_file_from_treeish
    treeish_object=self.raw_repo.get(treeish_string),
  File "/snap/git-ubuntu/122/usr/lib/python3/dist-packages/pygit2/repository.py", line 97, in get
    value = self.git_object_lookup_prefix(key)
ValueError: ubuntu/devel