NAV2013r2 - Service invoice adding unwanted discount

Hi All

I've searched all over and am struggling to spot an answer....

We currently use service contracts to create yearly invoices for some customers. It's a flat fee every year. Currently the first invoices generate fine, but for the second year onwards every line is given a 100% line discount. I can find lots of articles about putting a discount on service invoices, but we aren't adding any of this. We currently don't do any actual service work, but are just using service contracts to generate the invoices as they become due.

Has anyone had experience with using the service module to achieve this or know why NAV is trying to add 100% discount to everything after year 1?

Thanks