fastText backend fails when encountering an unknown subject URI

[osma]

Traceback:

Backend fasttext: creating fastText training file from documents
Traceback (most recent call last):
  File "/home/oisuomin/.local/share/virtualenvs/Annif-OYFUWV2R/bin/annif", line 11, in <module>
    load_entry_point('Annif', 'console_scripts', 'annif')()
  File "/home/oisuomin/.local/share/virtualenvs/Annif-OYFUWV2R/lib/python3.5/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-OYFUWV2R/lib/python3.5/site-packages/flask/cli.py", line 557, in main
    return super(FlaskGroup, self).main(*args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-OYFUWV2R/lib/python3.5/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-OYFUWV2R/lib/python3.5/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/oisuomin/.local/share/virtualenvs/Annif-OYFUWV2R/lib/python3.5/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-OYFUWV2R/lib/python3.5/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-OYFUWV2R/lib/python3.5/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-OYFUWV2R/lib/python3.5/site-packages/flask/cli.py", line 412, in decorator
    return __ctx.invoke(f, *args, **kwargs)
  File "/home/oisuomin/.local/share/virtualenvs/Annif-OYFUWV2R/lib/python3.5/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/home/oisuomin/git/Annif/annif/cli.py", line 122, in run_loaddocs
    proj.load_documents(documents)
  File "/home/oisuomin/git/Annif/annif/project.py", line 222, in load_documents
    self._load_documents_to_backends(documents, subjects)
  File "/home/oisuomin/git/Annif/annif/project.py", line 202, in _load_documents_to_backends
    backend.load_documents(documents, project=self)
  File "/home/oisuomin/git/Annif/annif/backend/fasttext.py", line 124, in load_documents
    self._create_train_file_from_documents(documents, project)
  File "/home/oisuomin/git/Annif/annif/backend/fasttext.py", line 107, in _create_train_file_from_documents
    method=self._write_train_file)
  File "/home/oisuomin/git/Annif/annif/util.py", line 19, in atomic_save
    method(obj, tempfilename)
  File "/home/oisuomin/git/Annif/annif/backend/fasttext.py", line 66, in _write_train_file
    labels = [cls._id_to_label(sid) for sid in subject_ids]
  File "/home/oisuomin/git/Annif/annif/backend/fasttext.py", line 66, in <listcomp>
    labels = [cls._id_to_label(sid) for sid in subject_ids]
  File "/home/oisuomin/git/Annif/annif/backend/fasttext.py", line 55, in _id_to_label
    return "__label__{:d}".format(subject_id)
TypeError: non-empty format string passed to object.__format__

Probably subject_id here is None, because the TSV document mentions a subject URI that is not in the SubjectIndex.