SAP Message WB2B_EXPENSE001 - The assignement to this document class ( &1 ) is not defined

Diagnosis
Expense processing was called from a document that the system does notrecognize. Expense processing can only be called from R/3 standarddocuments (such as sales order, delivery, billing document, purchaseorder, material document, invoice verification, and so on). This leadsto an internal error.

System Response
The system terminates processing.

Procedure
This is probably due to an error in a customer enhancement.

Procedure for System Administration
Please contact your system administrator.