Warning on metabox for relationship to multiple CPTs

Hi!
I'm getting this:
Warning: strtolower() expects parameter 1 to be string, array given in [...]/wp-includes/formatting.php on line 2126

This happens when I register a relationship to multiple CPT's and don't hide the meta_box.

    MB_Relationships_API::register([
        'id' => 'xxxx',
        'from' => [
            'post_type' => 'cpt1',
            'meta_box' => [
                'hidden'    => true,
            ]
        ],
        'to' => [
            'post_type' => ['cpt2','cpt3','cpt4','cpt5'],
            'meta_box' => [
                'title' => 'xxxx',
                'field_title' => 'xxxx',
            ],
        ],
    ]);

The warning disappears when I add 'hidden' => true to the 'to' section, or when I use only one cpt in the second section.

Any thoughts?

Thanx!