Invoice Number not appearing in Email

I was using the standard order_paid.html template and noticed that the invoice number was not appearing.

When looking in the forum I found this: https://forum.grabaperch.com/forum/02-07-2017-no-invoice-number-and-discount-code-in-email-template-perch-shop

but that didn't help. Neither orderInvoiceNumber or invoice_number worked. Everything else appears in the email except the invoice number.

Again, I'm using the standard order_paid.html template.