Should not have a 'None' option if type is 'Check boxes/radio buttons'

I think it should not provide a None (N/A) selection if the widget type for the field is 'Check boxes/radio buttons'
I think there is no logic to it. Apart from that, if you select the None (N/A) option you get

Notice: Undefined index: _none in taxonomy_field_validate() (line 1519 of /home/../modules/taxonomy/taxonomy.module).
Notice: Trying to get property of non-object in taxonomy_field_validate() (line 1519 of /home/../taxonomy/taxonomy.module).