BRF+ (MM) : Deriving a field in MARCPURCH using MARCBASIC

Hi,

I'm writing a simple derivation rule which:

     - Checks Article status - MMSTA on MARCBASIC

     - If it meets a specific criteria it must set the Purchasing Group EKGRP on MARCPURCH

This is how my Rule is set up:

Function name     : DERIVE_MARCPURCH

context                 : MARCBASIC & MARCPURCH

Rule                     : If MMSTA = 10 then change EKGRP based on a Decision Table.

I receive 2 errors:

1. Data Object MARCBASIC does not adhere to naming convention.

2. Data Object MARCBASIC in trigger function DERIVE_MARCPURCH not suitable to data model.

How can I get rule this to work?

Regards

Rafiq