Inventory Sales Report error

Hi, somehow when issuing a credit note to a customer a negative value of the Tax has been included in this report... Elsewhere (GL, Journal, Inventory, etc) everything is OK.

I invoiced for 50€ =>
41,32€ + 8,68€ (21% tax)

When I issued the credit note, everything is correct except in this report, where it is showing for this item:

Category Description Customer Qty Sales Cost Contribution
BC00226 B-220AURA-Rojo XXX   0    -8,68 0,00 -8,68

Tax report is OK:

Customer Credit Note1 19/02/2013 XXX -41,32 21,00 -8,68 IVA 21%

Thanks for any help !