When creating nested groups here, the settings start getting mangled up and showing up in the wrong place… any thoughts?

piklist( 'field', array(
    'type' => 'group',
    'label' => 'Ad Slots',
    'field' => '_ad_unit_ad_slots',
    'add_more' => true,
    'fields' => array(
        array(
            'type' => 'text',
            'field' => 'slot_id',
            'label' => 'Slot ID',
            'columns' => 12,
            'description' => 'The slot name',
        ),
        array(
            'type' => 'select',
            'field' => 'default_size',
            'label' => 'Default Ad Size',
            'columns' => 12,
            'choices' => array( 'option1' => 'Option 1', 'option2' => 'Option 2', 'option3' => 'Option 3')
        ),
        array(
            'type' => 'group',
            'field' => 'viewport_sizes',
            'add_more' => true,
            'fields' => array(
                array(
                    'type' => 'select',
                    'field' => 'viewport_size',
                    'label' => 'Viewport Size',
                    'columns' => 12,
                    'choices' => array( 'option1' => 'Option 1', 'option2' => 'Option 2', 'option3' => 'Option 3')
                ),
                array(
                    'type' => 'checkbox',
                    'field' => 'ad_sizes',
                    'label' => 'Ad Sizes',
                    'value' => '',
                    'columns' => 12,
                    'choices' => array( 'option1' => 'Option 1', 'option2' => 'Option 2', 'option3' => 'Option 3')
                )
            )
        )
    )
));