Filters dropdown not showing

I tried this code

<?php

$pod = pods( 'books' ); 

echo $pod->filters( array(
 'fields' => array( 'book_title', 'book_author', 'book_genre' )
) ); 

$pod->find(); 

while ( $pod->fetch() ) {
    echo "<li>" . $pod->display( 'book_title' ) . '</li>';
}

echo '<ul>'; 

echo $pod->pagination();

?>

It shows the search form with the search button but it is missing the filter dropdown for my custom fields. Is my code wrong?

The following code should display a dropdown menu containing my custom fields, right?

echo $pod->filters( array(
 'fields' => array( 'book_title', 'book_author', 'book_genre' )
) );

http://wordpress.org/plugins/pods/