Form does not render html input of RadioSelect

[Michael]

A form with the following field:
{{
send = forms.BooleanField(label=False, widget=forms.RadioSelect(attrs={'hidden':True}))
}}

when rendered as:
{{ form.as_table }}

will be rendered as:
{{
<tr>

<th></th>
<td>

<ul id="id_send">
</ul>

</td>

</tr>
}}

Note the lack of input widget. I am trying to create a hidden radio button, to store a boolean value that gets sent to the server. A CheckboxInput however does work, and is argueably better in this situation, but this may point to some underlying bug.