Missing '$template' parameter on emails

I'm trying to send an email after our IPN sends payment status updates, and email customers as well. I have the following code

$file_attachement = null; 
Mail::Send( (int)$email_order->id_lang, 
'order_conf', 
Mail::l('Order confirmation', (int)$email_order->id_lang), 
null, 
$email_customer->email, 
$email_customer->firstname.''.$email_customer->lastname, 
null, 
null, 
$file_attachement, 
null, 
_PS_MAIL_DIR_, false, (int)$email_order->id_shop );

And I get the following error

Missing '$template' parameter...smarty_internal_templatebase.php on line 177

The order_conf exists, and I can look at the Advanced Parameters->Email and see that its trying to send.

Prestashop Version is 1.7.5.1