Knack error when I run azdev linter

A weird error when I run azdev linter

cli.knack.cli: 'NoneType' object has no attribute 'startswith'
Traceback (most recent call last):
  File "c:\users\yfy\work\env\lib\site-packages\knack\cli.py", line 233, in invoke
    cmd_result = self.invocation.execute(args)
  File "c:\users\yfy\work\env\lib\site-packages\knack\invocation.py", line 225, in execute
    cmd_result = parsed_args.func(params)
  File "c:\users\yfy\work\env\lib\site-packages\knack\commands.py", line 148, in __call__
    return self.handler(*args, **kwargs)
  File "c:\users\yfy\work\env\lib\site-packages\knack\commands.py", line 255, in _command_handler
    result = op(client, **command_args) if client else op(**command_args)
  File "c:\users\yfy\work\azure-cli-dev-tools\azdev\operations\linter\__init__.py", line 118, in run_linter
    create_invoker_and_load_cmds_and_args(az_cli)
  File "c:\users\yfy\work\azure-cli\src\azure-cli-core\azure\cli\core\file_util.py", line 76, in create_invoker_and_load_cmds_and_args
    invoker.commands_loader.load_arguments()
  File "c:\users\yfy\work\azure-cli\src\azure-cli-core\azure\cli\core\__init__.py", line 490, in load_arguments
    c.argument('resource_group_name', resource_group_name_type)
  File "c:\users\yfy\work\azure-cli\src\azure-cli-core\azure\cli\core\commands\parameters.py", line 371, in argument
    if not self._applicable():
  File "c:\users\yfy\work\env\lib\site-packages\knack\arguments.py", line 153, in _applicable
    return self.command_loader.cli_ctx.invocation.data['command_string'].startswith(self.command_scope)
AttributeError: 'NoneType' object has no attribute 'startswith'

Version
knack, 0.8.0rc2
azdev, 0.1.29

Reproduce azdev linter, then start at knack/arguments.py in _applicable() and follow the command_string value through the knack invocation path. The traceback also points to azure/cli/core/file_util.py, azure/cli/core/__init__.py, and azure/cli/core/commands/parameters.py; done means the linter no longer raises this NoneType error.