any_other() records the wrong argument

[Qix-]

auto cli = (
	option("--help").set(show_help)
		% "Show this help message",
	(option("-J", "--threads") & value("num_threads", num_threads))
		% "The number of threads to use (default 0, meaning 'auto')",
	value("db_dir").set(dbdir).required(true).blocking(true)
		% "The database directory location",
	any_other(invalid_params)
);

I'm trying to use any_other to error out on unknown flags. In the above, if I run with ./my_prog --askdfjaksjd some_directory, invalid_params receives some_directory instead of the garbage flag. Is there a way to get the garbage flag in a list somewhere to give better error messages?