$ venv/bin/mitmdump --script ~/mitmproxy_arg_repro/plugin.py --set shout=1
[07:44:08.093] Loading script /mitmproxy_arg_repro/plugin.py
[07:44:08.094] error in script /mitmproxy_arg_repro/plugin.py
Traceback (most recent call last):
  File "/src/mitmproxy/mitmproxy/addons/script.py", line 124, in loadscript
    ctx.master.addons.register(ns)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
  File "/src/mitmproxy/mitmproxy/addonmanager.py", line 195, in register
    self.master.options.process_deferred()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/src/mitmproxy/mitmproxy/optmanager.py", line 360, in process_deferred
    value = self._parse_setval(self._options[optname], value.val)
  File "/src/mitmproxy/mitmproxy/optmanager.py", line 405, in _parse_setval
    raise exceptions.OptionsError(
        'Boolean must be "true", "false", or have the value omitted (a synonym for "true").'
    )
mitmproxy.exceptions.OptionsError: Boolean must be "true", "false", or have the value omitted (a synonym for "true").
[07:44:08.095] HTTP(S) proxy listening at *:8080.
Error logged during startup, exiting...