Jupytext --set-kernel fails in version 1.2.3 on .markdown files

[mwouts]

For instance, jupytext --set-kernel elegant-scipy-kernel markdown/* fails with this message:

[jupytext] Reading markdown/acknowledgements.markdown
[jupytext] Updating notebook metadata with '{"kernelspec": {"name": "elegant-scipy-kernel", "language": "python", "display_name": "elegant-scipy-kernel"}}'
[jupytext] Writing markdown/acknowledgements.markdown in format markdown:markdown (destination file replaced)
[jupytext] Reading markdown/ch1.markdown
[jupytext] Updating notebook metadata with '{"kernelspec": {"name": "elegant-scipy-kernel", "language": "python", "display_name": "elegant-scipy-kernel"}}'
[jupytext] Writing markdown/ch1.markdown in format markdown:markdown (destination file replaced)
[jupytext] Error: unsupported operand type(s) for +: 'NoneType' and 'str'

[mwouts]

With the version 1.2.4-dev of Jupytext, the full error is

[jupytext] Reading markdown/acknowledgements.markdown
[jupytext] Updating notebook metadata with '{"kernelspec": {"name": "elegant-scipy-kernel", "language": "python", "display_name": "elegant-scipy-kernel"}}'
[jupytext] Writing markdown/acknowledgements.markdown in format markdown:markdown (destination file replaced)
[jupytext] Reading markdown/ch1.markdown
[jupytext] Updating notebook metadata with '{"kernelspec": {"name": "elegant-scipy-kernel", "language": "python", "display_name": "elegant-scipy-kernel"}}'
[jupytext] Writing markdown/ch1.markdown in format markdown:markdown (destination file replaced)
Traceback (most recent call last):
  File "/home/marc/jupyter-book-experiments/jupytext/jupytext/header.py", line 65, in encoding_and_executable
    cell.source.encode('ascii')
UnicodeEncodeError: 'ascii' codec can't encode character '\u2014' in position 310: ordinal not in range(128)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/marc/miniconda3/envs/python3/bin/jupytext", line 11, in <module>
    load_entry_point('jupytext', 'console_scripts', 'jupytext')()
  File "/home/marc/jupyter-book-experiments/jupytext/jupytext/cli.py", line 228, in jupytext
    exit_code += jupytext_single_file(nb_file, args, log)
  File "/home/marc/jupyter-book-experiments/jupytext/jupytext/cli.py", line 387, in jupytext_single_file
    write(notebook, nb_dest, fmt=dest_fmt)
  File "/home/marc/jupyter-book-experiments/jupytext/jupytext/jupytext.py", line 333, in write
    write(nb, stream, version=version, fmt=fmt, **kwargs)
  File "/home/marc/jupyter-book-experiments/jupytext/jupytext/jupytext.py", line 338, in write
    content = writes(nb, version=version, fmt=fmt, **kwargs)
  File "/home/marc/jupyter-book-experiments/jupytext/jupytext/jupytext.py", line 307, in writes
    return writer.writes(notebook, metadata)
  File "/home/marc/jupyter-book-experiments/jupytext/jupytext/jupytext.py", line 128, in writes
    header = encoding_and_executable(nb, metadata, self.ext)
  File "/home/marc/jupyter-book-experiments/jupytext/jupytext/header.py", line 67, in encoding_and_executable
    lines.append(comment + _UTF8_HEADER)
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'