SAP Message >R163 - Billing type reversal &1 or orig. &2 not from cat. IS-U billing request

Diagnosis
The billing type of the original billing document &v2& or the reversalbilling document &v1& does not belong to the category 'IS-U billingrequest'.
If the billing type of the original billing document has been assignedto the billing category 'IS-U billing request' then the reversalbilling type must also be assigned to this category. The reverse isalso true.
INCLUDE FICA_NA_CUST OBJECT DOKU ID TX