Blank email subject and body

I’m getting a blank email when I test the plugin.

It’s the email that is sent to the referrer once the person they referred completes a purchase.

Here is what it looks like in Gmail:
https://dev.vstbuzz.com/wp-content/uploads/2016/05/vstbuzz-email.png

I’m using the WooCommerce Pretty Emails plugin as well but I get the same blank email even when that is disabled.

Please advise.

https://www.ads-software.com/plugins/refer-a-friend-for-woocommerce-by-wpgens/