Web services & empty fields & lookup problem

Hello,

Using web services in NAV 2013 to create invoices, for which purpose I have the required tables mapped to pages & exposed thorugh web services. Now, my problem is that when I want to e.g. pass empty value (null or empty string) in invoice's "Sell-to Address2", NAV tries to look up that value from the customer of the invoice. How can I disable this behaviour?

Thanks!