Add CSS to AMPs

Is there a way to add CSS code to AMPosts only? I tried to add this to the functions.php file:
function custom_post_head() {
if (is_single()) { ?>
<style amp-custom>
.amp-wp-meta.amp-wp-byline {display: none;}
</style>
<?php }}
add_action(‘wp_head’, ‘custom_post_head’);

But it doesn’t work!