Dependent: Form items don't hide if nothing is selected in the radio item they depend on (e.g. no default_value)

When I have a radio form item where it isn't applicable to define a default_value then all form items that depend on the radio are shown regardless of their dependency values. Upon selecting an option in the radio the dependency works like expected (items with different dependency values are hidden).

In case I haven't used dependent correctly - here are the form items:

$form['source'] = array(
  '#type' => 'radios',
  '#title' => t('Choose the object'),
  // Often $form_state['taglist']->source['source'] is not set so default_value is not present.
  '#default_value' => $form_state['taglist']->source['source'] ? $form_state['taglist']->source['source'] : NULL,
  '#options' => $options['source'],
);

foreach ($options['source'] as $viewname) {
  $form[$viewname] = array(
    '#type' => 'select',
    '#title' => t('Displays for view !viewname', array('!viewname' => $viewname)),
    '#default_value' => $form_state['taglist']->source['display'] ? $form_state['taglist']->source['display'] : 'default',
    '#options' => $options['display'][$viewname],
    // Works when an option is selected in the radio form 'source'.
    '#process' => array('ctools_dependent_process'),
    '#dependency' => array('radio:source' => array($viewname)),
  );
}