Radio Buttons do not receive default value

As would be defined according to the FAPI, a default value (i.e. of 'two') doesn't work:

'sample_radios' => array(
  '#weight' => '3',
  '#default_value' => 'two',
  '#options' => array(
    'one' => t('one'),
    'two' => t('two'),
    'three' => t('three'),
  ),
  '#type' => 'radios',
  '#title' => t('Sample radios'),
);

Nor does it work as in the TableForm module's own example with a default set (i.e. of 'array(2)'):

'my_radios' => array(
  '#type'           => 'radios',
  '#title'          => t('Radios'),
  '#options'        => array(
    t('Apple'),
    t('Orange'),
    t('Banana'),  
  ),
  '#default_value'  => array(2),
),