Get all comments associated with a specific page ID (comment_post_ID)

I try to get all comments of a specific post, but it is not working like the way I do want. I use $assignment->ID to get all comments for a specific page ID. Even when I change $assignment->ID to 101 it is not working and he still shows all the comments of all posts.

foreach($assignments as $assignment) {

    echo $assignment->post_title;
    $args = array(
        'number' => 0,
        'status' => 'approve',
        // shows all comments, but it shouldn't
        'comment_post_ID' => $assignment->ID
    $comments = get_comments( $args );
    if ( $comments ) {
        foreach ( $comments as $comment )   {
            echo '<li>';
            echo $comment->comment_content;
            echo '</li>';


I think I miss an important $arg, but I’m not sure which one. It doesn’t matter what I do, all comments are showing up everytime..