[Plugin: Developer's Custom Fields] slt_cf_field_value() returns nothing

I’ve defined my custom fields as per the example in the documentation, only slightly modified for my post type “Book”:

if ( function_exists( 'slt_cf_register_box') )
	add_action( 'init', 'register_my_custom_fields' );

function register_my_custom_fields() {
	slt_cf_register_box( array(
		'type'		=> 'post',
		'title'		=> 'Additional info',
		'id'		=> 'book-box',
		'context'	=> 'normal',
		'priority'	=> 'high',
		'fields'	=> array(
			array(
				'name'			=> 'reviews',
				'label'			=> 'Reviews',
				'type'			=> 'wysiwyg',
				'scope'			=> array( 'book' ),
				'capabilities'	=> array( 'edit_posts' )
			),
			array(
				'name'			=> 'extras',
				'label'			=> 'Extras',
				'type'			=> 'wysiwyg',
				'scope'			=> array( 'book' ),
				'capabilities'	=> array( 'edit_posts' )
			)
		)
	));
}

The data is stored/updated correctly in the back-end.

When I try to access it on the front-end, in my single view (single-book.php) it shows nothing. I’m using:
<?php echo '<p>Reviews: ' . slt_cf_field_value( "reviews" ) . '</p>'; ?>

Am I doing it wrong?

http://wordpress.org/extend/plugins/developers-custom-fields/