Malformed tag in HTML header

Affected Version: 0.6.0

Issue: The inserted <link> tag is missing a closing slash <link …/>

Affected File: amp/includes/amp-frontend-actions.php:27

Solution: The missing slash needs to be added to the printf command in the aforementioned file: '<link rel="amphtml" href="%s" />'

Workaround: Add the following to the functions.php of your theme until it is fixed in one of the upstream releases.

add_action( 'wp_head', 'afe_amp_frontend_add_canonical', 9);
// fix missing end of html tag <link /> in amp plugin
function afe_amp_frontend_add_canonical() {
        remove_action( 'wp_head', 'amp_frontend_add_canonical');
        if ( false === apply_filters( 'amp_frontend_show_canonical', true ) ) {
          return;
        }

        $amp_url = amp_get_permalink( get_queried_object_id() );
        printf( '<link rel="amphtml" href="%s" />', esc_url( $amp_url ) );
}