How can I noindex my archive pages?

I am using the following snippet in my functions.php.

function add_noindex() {
    if(is_archive()) {
        echo '<meta name="robots" content="noindex,follow" />';
    }
}
add_action( 'wp_head', 'add_noindex' );

I thought this would noindex all my archive pages, but it is not working. My archive page code looks like this:

<?php get_header(); ?>
<h1><?php single_cat_title();?></h1>

<?php if (have_posts()) : while(have_posts()) : the_post();?>
    <h3><?php the_title();?></h3>
    <?php the_excerpt();?>
    <a href="<?php the_permalink();?>">Read More</a>
<?php endwhile; 
endif;?>

Any ideas? Thank you in advance!