py4web problem using CheckboxWidget with list:reference field

[davidmanns]

Consider a simple example:

db.define_table('mailings',
Field('listname'),
format='%(listname)s'
)

db.define_table('people',
Field('first'),
Field('last'),
Field('lists', 'list:reference mailings', widget=CheckboxWidget)
)

displaying a CRUD form (using the Edit button in Grid) fails, see below.
removing the widget specification works, but on the desktop the widget displays a scrolling list, where one must ctrl-click to toggle individual selections. The above works on web2py, displaying a nice list of checkboxes. Much better UI on desktop.

Traceback (most recent call last):
File "C:\Users/David/SkyDrive/py4web\py4web\core.py", line 163, in safely
return func()
File "C:\Users/David/SkyDrive/py4web\py4web\core.py", line 503, in
lambda: data.xml(), default=lambda: self._to_html(self._to_unicode(data))
File "C:\Users/David/SkyDrive/py4web\py4web\utils\form.py", line 1022, in xml
return self.structure.xml()
File "C:\Users/David/SkyDrive/py4web\py4web\utils\form.py", line 1016, in structure
return self.helper()["form"]
File "C:\Users/David/SkyDrive/py4web\py4web\utils\form.py", line 937, in helper
helper = self.param.formstyle(
File "C:\Users/David/SkyDrive/py4web\py4web\utils\form.py", line 431, in call
control = field.widget(table, vars)
TypeError: CheckboxWidget() takes no arguments