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Try `nargs='?'` or try providing a `default` along with the '*'.
Including your ARGUMENT action in the error message is intentional.
The test for this error message is:
 required_actions = []
 for action in self._actions:
 if action not in seen_actions:
 if action.required:
Originally the code just checked if `positionals` was empty. That was the list of positional Actions. Actions were popped as they were parsed. Now it maintains a set `seen_actions`, and checks the `required` attribute. This test applies to both positionals and optionals.
For optionals, `required` is set by the programmer. But for positionals it is set with:
 def _get_positional_kwargs
 ...
 # mark positional arguments as required if at least one is
 # always required
 if kwargs.get('nargs') not in [OPTIONAL, ZERO_OR_MORE]:
 kwargs['required'] = True
 if kwargs.get('nargs') == ZERO_OR_MORE and 'default' not in kwargs:
 kwargs['required'] = True
So for '?' argument, required is False. But for '*', it must also have a 'default' parameter (not just the default None).
So the proposed patch is overriding the 'required' value that was set during 'add_argument'. And issuing this error message is the main purpose of the 'required' attribute.
I would not implement this patch. 
But it would be a good idea to check if this method of setting the required attribute has been discussed in other bug/issues. (There is an open issue concerning how the 'required' is set/or not for the subparsers positional.)
Off hand I don't see anything about this in the documentation. Maybe that's what needs to be patched. (It's easier and safer to change the documentation than the working code. Backward compatibility is a major concern when changing the code.)