Error message when trying to build a Jupyter-book with ```- url: https://ubicenter.org \n title: UBI Center home``` (see https://github.com/executablebooks/jupyter-book/issues/764)

sphinx-err-pb292o6k.log

# Sphinx version: 2.4.4
# Python version: 3.7.7 (CPython)
# Docutils version: 0.16 release
# Jinja2 version: 2.11.2
# Last messages:

# Loaded extensions:
Traceback (most recent call last):
  File "/home/mghenis/anaconda3/lib/python3.7/site-packages/jupyter_book/sphinx.py", line 209, in build_sphinx
    keep_going=keep_going,
  File "/home/mghenis/anaconda3/lib/python3.7/site-packages/sphinx/application.py", line 269, in __init__
    self.events.emit('config-inited', self.config)
  File "/home/mghenis/anaconda3/lib/python3.7/site-packages/sphinx/events.py", line 99, in emit
    results.append(callback(self.app, *args))
  File "/home/mghenis/anaconda3/lib/python3.7/site-packages/jupyter_book/toc.py", line 192, in update_indexname
    app.config["master_doc"] = _no_suffix(toc["file"])
KeyError: 'file'