[web2py] using radio widget can't receive the empty submit

[Dan]

Hello,

I'm testing empty submit with the following two forms,  one is
dropdown list and the other one is the checkradio box with v1.99.4.

The dropdown list with empty select can successfully submit and got
the expected responses.  But using checkradio widget, empty submit
doesn't work and no response at all.

Is this a bug for radio.widget or something else?  Many thanks.

In my controller
test.py

form = SQLFORM.factory(Field('Form', requires=IS_IN_SET([],
multiple='multiple')))
    if form.accepts(request.vars,session,formname='form'):
        response.flash = "Form Response"
        message = form.vars

xform = SQLFORM.factory(Field('FormX', requires=IS_IN_SET([],
multiple='multiple'), widget = SQLFORM.widgets.radio.widget))
    if xform.accepts(request.vars,session,formname='xform'):
        response.flash = "Form X Response"
        message = xform.vars
return dict(xform=xform,form=form)

In my viewer
test.html

{{=form}}
{{=xform}}