#states cannot disable/enable radios and checkboxes

I am trying to use #states => array( 'enabled' => ... ) or #states => array( 'disabled' => ... ) on form elements of #type 'radios' or 'checkboxes'.

When the state change conditions apply, the disabled attribute gets set on a div around the radio buttons / checkboxes. The radio buttons / checkboxes themselves are not enabled or disabled.