mkdocs-multirepo-plugin YAML validation fails for imported repo with `imported

I'm trying to setup the "Development in Imported Repos" feature as documented here: https://github.com/jdoiro3/mkdocs-multirepo-plugin#%CE%B2-development-in-imported-repos-beta.

Unfortunately I'm always getting the following exception. I'm using the latest release v0.6.1.

Traceback (most recent call last):
  File "/usr/local/bin/mkdocs", line 8, in <module>
    sys.exit(cli())
             ^^^^^
  File "/usr/local/lib/python3.11/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/__main__.py", line 247, in build_command
    cfg = config.load_config(**kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/config/base.py", line 363, in load_config
    errors, warnings = cfg.validate()
                       ^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/config/base.py", line 228, in validate
    run_failed, run_warnings = self._validate()
                               ^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/config/base.py", line 186, in _validate
    self[key] = config_option.validate(value)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/config/config_options.py", line 147, in validate
    return self.run_validation(value)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/config/config_options.py", line 939, in run_validation
    self.load_plugin_with_namespace(name, cfg)
  File "/usr/local/lib/python3.11/site-packages/mkdocs/config/config_options.py", line 975, in load_plugin_with_namespace
    return (name, self.load_plugin(name, config))
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/config/config_options.py", line 1012, in load_plugin
    errors, warns = plugin.load_config(
                    ^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/plugins.py", line 108, in load_config
    return self.config.validate()
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/config/base.py", line 228, in validate
    run_failed, run_warnings = self._validate()
                               ^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/config/base.py", line 186, in _validate
    self[key] = config_option.validate(value)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/config/config_options.py", line 147, in validate
    return self.run_validation(value)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/mkdocs/config/config_options.py", line 247, in run_validation
    if not isinstance(value, self._type):
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/typing.py", line 472, in __instancecheck__
    raise TypeError(f"{self} cannot be used with isinstance()")
TypeError: typing.Union cannot be used with isinstance()