Cannot hide relationship

I'm trying to create a relationship where the "to" side is hidden:

define ( 'TYPE_PRODUCT',        'product' );  
define ( 'TYPE_INDUSTRY',       'industries' );
define ( 'PRODUCT_TO_INDUSTRY', 'product_to_industry');

    MB_Relationships_API::register( array(
        'id'   => PRODUCT_TO_INDUSTRY,
        'from' => array(
            'object_type' => 'post',
            'post_type'   => TYPE_PRODUCT,
            'meta_box'    => array(
                'title'         => 'Linked Industries',
            ),
        ),
        'to' => array(
            'object_type' => 'post',
            'post_type'   => TYPE_INDUSTRY,
            'meta_box'    => array(
                'hidden'        => true,
            ),
        ),
    ) );

But I cannot get the "to" side to be hidden. I've tried moving the 'hidden' parameter around and using 'true', without success.

Can you assist? thanks.