Actual cost process error

--> On running the Actual Cost process, when we view errors, the following error is displayed "Cannot find the cost for the item 0300000002 in warehouse 010P". 

-->   Also while updating OPM subledger in test mode, we get the following error:
"No cost for Item:0300000002, Whse:010P, Cost Whse:010P, Date:03-JAN-2012."

The item (0300000002) in question is a by product. By-Products are not expected to be costed and hence we expect the item to be excluded from Actual cost calculations and during subleger update. 

Regards,

Birju