wp_get_attachment_image not working with get_sub_field (ACF)

I’m using the Advanced Custom Fields Plugin function get_sub_field to pass the URL to wp_get_attachment_image but it’s simply returning blank:

<?php
$url = get_sub_field( 'profile_picture' );
echo wp_get_attachment_image( $url, 'full' );
?>

PS I can’t post in the ACF forums here because they’re not monitored and I can’t on the official website because something is wrong with my account so I can’t sign in anyway. So please don’t bother scolding me for not getting support there.