do_enclose() throws warnings when content_type is text/html and not audio or video

[tamas_dxw]

cron calls do_pings()/do_all_pings() which in turn goes on to do_enclose().

PHP Warning: strpos() expects parameter 1 to be string, array given in /var/vhost/site/wp-includes/functions.php on line 505

$url is valid, $type is "text/html; charset=UTF-8", $len is valid as well.
$allowed_types is array(0=>video, 1=>audio)

server environment:
PHP 5.5.3-1ubuntu2.1
Apache 2.4.6
running WP 3.8.1, no multisite