class 'RWMB__Field' not found

Hello,

Currently, I load the metabox (v4.8.5) in my theme via functions.php like so:

require get_template_directory() . '/lib/meta-box/meta-box.php'; 
require get_template_directory() . '/lib/meta-box-group/meta-box-group.php'; 
include 'meta.php';

Next I add the following to my Meta definitions in meta.php

$meta_boxes[] = array(
	'id'		=> 'group_field',
	'title'		=> 'Teasers',
	'pages'		=> array( 'feature'),
	'fields'	=> array(
			array(
				'id'     => 'standard',
				// Gropu field
				'type'   => 'group',
				// Clone whole group?
				'clone'  => true,
				// Drag and drop clones to reorder them?
				'sort_clone' => true,
				// Sub-fields
				'fields' => array(
					array(
						'name' => __( 'Track name', 'rwmb' ),
						'id'   => 'text',
						'type' => 'text',
					),
					array(
						'name' => __( 'Release Date', 'rwmb' ),
						'id'   => 'date',
						'type' => 'date',
					),
					array(
						'name'    => __( 'Genre', 'rwmb' ),
						'id'      => 'genre',
						'type'    => 'select_advanced',
						'options' => array(
							'pop'  => __( 'Pop', 'rwmb' ),
							'rock' => __( 'Rock', 'rwmb' ),
						),
					),
				),
			),
		),
	);

Yet I get the following error?

"Warning: call_user_func() expects parameter 1 to be a valid callback, class 'RWMB__Field' not found in /www/koordinates/wp-content/themes/theme/lib/meta-box/inc/meta-box.php on line 221"

I'm using all the latest builds as far as I know… any ideas?