Internal error Production Orders

Good Day Experts,

I have an issue where created a UDF on Production Orders to update Warehouse.

The UDF was created on the Production Order Header, when updating/changing the warehouse I get the following error:

Internal error 'Items - Warehouse' (OITW)(-2118) occured [Message 131-183.

i did try the query below what i found on a previous discussion but it did not fix the issue:

DECLARE @ItemCode nvarchar(20)

SELECT @ItemCode = $[$37.4.0]

IF  @ItemCode = ''

     SELECT $[$37.10.0]

ELSE

     SELECT $[OWOR.Warehouse]

Query for UDF:

SELECT $[OWOR.U_WHS] FROM OWOR T0

SAP Version 9.1 PL.08.

Please advise.

Regards,

Izak