Sphinx error on building user's guide

Summary

It may depend on my build environment, but I report it.

When building the user's guide on HEAD(d584e3f0), the following error occurs:

  :
reading sources... [ 94%] utils
reading sources... [ 97%] what_glasgow_exts_does
reading sources... [100%] win32-dlls

looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [  2%] 8.10.1-notes

Encoding error:
'ascii' codec can't encode character u'\u27e8' in position 132: ordinal not in range(128)
The full traceback has been saved in /tmp/sphinx-err-Y4GUm5.log, if you want to report the issue to the developers.
docs/users_guide/ghc.mk:16: recipe for target 'docs/users_guide/build-html/users_guide/index.html' failed

It worked correctly last month in my environment.

Steps to reproduce

% cd docs/users_guide
% make html

Expected behavior

Generating html documents.

Environment

• Building source: Current master's HEAD (d584e3f0)

• GHC version used: Building with GHC 8.6.5

• Sphinx version used: v1.6.7

• Operating System: Ubuntu 18.04 LTS

• System Architecture: x86_64