How to force a mapping set value

Summary:

TAB Account rules for categories in procurement

Content (required):

I am creating account rule and mapping set to derive natural account for requisitions based on item category for expense purchases. All requestors have a default expense account setup and then segments are updated based on account rules and mapping set. This works fine, but when I create a new category and do not have any value in mapping set for new category then there is no error and default account from employee setup is used. I just want to error out if there is no mapping set definition for category.