Cannot Create A New Mail Template For Vendors

I'm trying to replace the mail template used for notifying vendors when an order status has changed to "Processed".

I understand that for admins and vendors the templates required are in the path:

design/backend/mail/templates/orders/order_notification.tpl

So what I am doing is creating a new file like the following:

design/backend/mail/templates/orders/custom_set_pickup.tpl

And also changing the "fn_order_notification" function so that the section that sends to the vendor (delimited by if ($notify_vendor == true)) has a different 'tpl' parameter like the following:

Mailer::sendMail(array(
                    'to' => 'company_orders_department',
                    'from' => 'default_company_orders_department',
                    'data' => array(
                        'order_info' => $order_info,
                        'shipments' => $shipments,
                        'use_shipments' => $use_shipments,
                        'order_status' => fn_get_status_data($order_info['status'], STATUSES_ORDER, $order_info['order_id'], $company_lang_code),
                        'payment_method' => fn_get_payment_data($payment_id, $order_info['order_id'], $company_lang_code),
                        'status_settings' => $status_settings,
                        'profile_fields' => fn_get_profile_fields('I', '', $company_lang_code),
                        'secondary_currency' => $secondary_currency,
                        'go_to_link' => $go_to_link
                    ),
                    'tpl' => 'orders/custom_set_pickup.tpl',
                    'company_id' => $order_info['company_id'],
                ), 'A', $company_lang_code);

However this fails for some reason.

(For testing purposes the contents of this new mail template are the exact same as the original one 'order_notification.tpl')

I'd appreciate any help.

Thanks

I found out the reason.

Along with "custom_set_pickup.tpl" another file named "custom_set_pickup_subj.tpl" must also be created.