Error when trying to change default actions for messages if acceptable_aliases is set to []

I've got a small instance of mailman with 4 lists running. On two of them, I can change the default actions for messages no problem. On the other two, when I try to change the default action for members, I get the following (redacted slightly to disguise the production instance in question):

Request Method: 	POST
Request URL: 	http://REDACTED.EXAMPLE.COM/mailman3/lists/LIST.REDACTED.EXAMPLE.COM/settings/message_acceptance
Django Version: 	1.7.10
Exception Type: 	UnboundLocalError
Exception Value: 	

local variable 'HTTPError' referenced before assignment

Exception Location: 	/home/mailman/mailman-bundler/eggs/postorius-1.0.1-py2.7.egg/postorius/views/list.py in list_settings, line 787
Python Executable: 	/home/mailman/venv/bin/python2

(...)
Traceback Switch to copy-and-paste view

    /home/terri/mailman-bundler/eggs/Django-1.7.10-py2.7.egg/django/core/handlers/base.py in get_response

                            response = wrapped_callback(request, *callback_args, **callback_kwargs)

         ...
    ▶ Local vars
    /home/mailman/mailman-bundler/eggs/postorius-1.0.1-py2.7.egg/postorius/auth/decorators.py in wrapper

                    return fn(*args, **kwargs)

         ...
    ▶ Local vars
    /home/mailman/mailman-bundler/eggs/postorius-1.0.1-py2.7.egg/postorius/views/list.py in list_settings

                        except HTTPError as e:

         ...
    ▶ Local vars 

It was not initially clear to me what made those two lists different, but I've figured out now that it's because the acceptable aliases were set to [] (that is, a string containing an open square bracket followed by a close square bracket). Presumably some sort of parse error is occurring that we should handle appropriately.