add value to database on register_user

Hey guys, I’m using newsletter plugin, and I’d like to start automatically adding all the newly registered users to the mailing list.
I’ve written a function that I believe should work, but for some reason, it doesn’t. Could you have a look?

add_action('user_register', 'add_to_newsletter');

function add_to_newsletter($user_id){
	$user = get_user_by('id', $user_id);
	$email = $user->user_email;
	global $wpdb;
		$wpdb->insert($wpdb->prefix . ‘newsletter’, array(‘email’=>$email));
}