Deny sales invoice if customer is invalid

i'm sure this is an easy fix, but i'm not sure how/what to do...

I have a Small  VS C# console app that creates invoices in SAGE 50 for me.

When i enter a existing customer name all is good, but if i make a mistake and the customer does not exist, it just creates the invoice anyways using the name i put.

the invoice comes out posted as "paid" and its not linked to a customer account its just "there"

is there an option to insure a invoice cannot be generated if the customer does not exist?