Error

Command /home/docs/checkouts/readthedocs.org/user_builds/abejgonza-riscv-boom/envs/stable/bin/python -m sphinx -T -E -W --keep-going -b epub -d _build/doctrees -D language=en . _build/epub failed: Running Sphinx v1.8.5 [READTHEDOCS] HOSTNAME = build-16087570-project-269072-abejgonza-riscv-boom [READTHEDOCS] HOME = /home/docs [READTHEDOCS] NO_COLOR = 1 [READTHEDOCS] READTHEDOCS = True [READTHEDOCS] READTHEDOCS_PROJECT = abejgonza-riscv-boom [READTHEDOCS] PATH = /home/docs/checkouts/readthedocs.org/user_builds/abejgonza-riscv-boom/envs/stable/bin:/home/docs/.asdf/shims:/home/docs/.asdf/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin [READTHEDOCS] LANG = C.UTF-8 [READTHEDOCS] READTHEDOCS_LANGUAGE = en [READTHEDOCS] DEBIAN_FRONTEND = noninteractive [READTHEDOCS] READTHEDOCS_VERSION = stable [READTHEDOCS] PWD = /home/docs/checkouts/readthedocs.org/user_builds/abejgonza-riscv-boom/checkouts/stable/docs [READTHEDOCS] DOCUTILSCONFIG = /home/docs/checkouts/readthedocs.org/user_builds/abejgonza-riscv-boom/checkouts/stable/docs/docutils.conf [TESTING!] b'v3.1.2\n' loading translations [en]... done making output directory... building [mo]: targets for 0 po files that are out of date building [epub]: targets for 1 source files that are out of date updating environment: 1 added, 0 changed, 0 removed reading sources... [100%] index looking for now-outdated files... none found pickling environment... done checking consistency... done preparing documents... done writing output... [100%] index generating indices... genindex writing additional pages... copying static files... done copying extra files... done writing mimetype file... writing META-INF/container.xml file... writing content.opf file... WARNING: unknown mimetype for _static/DUMMY, ignoring writing nav.xhtml file... writing toc.ncx file... writing RISCV-BOOM.epub file... build finished with problems, 1 warning. Updating searchtools for Read the Docs search...