Use filter “og_image_init”: with CPT

Use filter “og_image_init”: for my CPT. I’ve tried the below but it’s not working. I’d just like to always use a default image for the event CPT. Is this possible?

/* Default image for Events FB sharing */
add_filter('og_image_init', 'my_og_image_init');
function my_og_image_init($images)
{
    if ( is_singular('event')) {
        $images[] = 'https://mywebsite.co.uk/wp-content/uploads/2016/01/my-picture.jpg';
    }
    return $images;
}

Many thanks for any help.