Section nav error: TypeError: unsupported operand type(s) for +: 'int' and 'str'

• [ x] I’ve read the contribution guidelines and agree with them

I’ve found a bug and checked that …

• [ x] … the problem doesn’t occur with the default MkDocs template
• [x ] … the problem is not in any of my customizations (CSS, JS, template)
• [ x] … the documentation does not mention anything about my problem
• [ x] … there are no open or closed issues that are related to my problem

Description

I get the following error when activating section based navigation with mkdocs-material-6.1.4+insiders-1.9.0 in a minimal website.

ERROR   -  Exception in callback <bound method LiveReloadHandler.poll_tasks of <class 'livereload.handlers.LiveReloadHandler'>>
Traceback (most recent call last):
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/tornado/ioloop.py", line 907, in _run
    return self.callback()
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/livereload/handlers.py", line 69, in poll_tasks
    filepath, delay = cls.watcher.examine()
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/livereload/watcher.py", line 119, in examine
    func()
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/mkdocs/commands/serve.py", line 136, in builder
    build(config, live_server=live_server, dirty=dirty)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/mkdocs/commands/build.py", line 285, in build
    _build_theme_template(template, env, files, config, nav)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/mkdocs/commands/build.py", line 108, in _build_theme_template
    output = _build_template(template_name, template, files, config, nav)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/mkdocs/commands/build.py", line 87, in _build_template
    output = template.render(context)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/jinja2/asyncsupport.py", line 76, in render
    return original_render(self, *args, **kwargs)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/jinja2/environment.py", line 1008, in render
    return self.environment.handle_exception(exc_info, True)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/jinja2/environment.py", line 780, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/jinja2/_compat.py", line 37, in reraise
    raise value.with_traceback(tb)
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/material/404.html", line 4, in top-level template code
    {% extends "main.html" %}
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/jinja2/environment.py", line 1005, in render
    return concat(self.root_render_func(self.new_context(vars)))
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/material/404.html", line 14, in root
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/material/main.html", line 14, in root
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/material/base.html", line 126, in root
    <main class="md-main" data-md-component="main">
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/material/base.html", line 360, in block_site_nav
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/material/partials/nav.html", line 59, in root
  File "/usr/local/Caskroom/miniconda/base/envs/rapids202008/lib/python3.7/site-packages/material/partials/nav-item.html", line 38, in root
    </nav>
TypeError: unsupported operand type(s) for +: 'int' and 'str'

Expected behavior

Load the website without errors and with section navigation

Actual behavior

The website is not rendered

Steps to reproduce the bug

1. Install mkdocs-material-6.1.4+insiders-1.9.0
2. Setup the mkdocs.yml described below
3. Create a test file under /docs/test.md with this line # My Test
4. mkdocs serve

Package versions

• Python: 3.7.3 python --version
• MkDocs: 1.1.2 mkdocs --version
• Material: Version: 6.1.4+insiders.1.9.0 pip show mkdocs-material | grep -E ^Version

Project configuration

site_name: Test
theme:
  name: material
  features:
    - navigation.sections
nav:
  - Others:
    - Team: test.md

System information

• OS: MacOS 10.15.7

• Browser: Firefox 82.0.2

[squidfunk]

We added Jinja as an explicit dependency in https://github.com/squidfunk/mkdocs-material/commit/905c8b77c689e459b37b64564d81775d5ca60879, released as part of 7.3.3, see here:

https://github.com/squidfunk/mkdocs-material/blob/1316d8d184b63ddb9e308956b2599c7320709bf7/requirements.txt#L22

[binwiederhier]

For the Googlers, I ran into the same issue here: https://github.com/binwiederhier/ntfy/actions/runs/1550547757 and fixed it with an explicit jinja2 dependency, here: https://github.com/binwiederhier/ntfy/commit/ab3cc47e272aa0d21f8b3fd134265611916b6416#diff-4d7c51b1efe9043e44439a949dfd92e5827321b34082903477fd04876edb7552R4

Thanks @liuhualh for the hint and thanks @squidfunk for a freaking awesome piece of software (which has docs here: https://ntfy.sh/docs).