Problem with Vendor Rebate Process

Hi gurus,

We have a problem with vendor rebate process. This is our process:

Rebate Process:

1. XK02 - Changing a Vendor Master Record (Activate "Subsequent Settlement" and "Sebseq. sett. index" in Purchasing view)

2. MEB1 - Creating a Rebate Arrangement

3. ME21N - Creating a Purchase Order

4. MIGO - Processing the Goods Receipt for the Purchase Order

5. MIRO - Processing the Incoming Invoice on the Basis of a Purchase Order

6. MEB4 - Creating a Final Settlement

In sixth step system alarms with next message: "G/L account xxxx requires a valid tax code", we think is about determination tax material relationated with dummy material for settlement rebate.

Any help??? I will revvard you!

Thank you!!!