whats wrong with this Sum code...?!

[redblaze]

=Sum([txtSubtotal]+[txtShippingCost])-[txtDiscount]

i have entered this code as the control source of the field "GrandTotal" on my invoice form, for some reason it displays #Error. i have tried everything i can think of but cant get it to work? thanks for your help.

 

[Rich]

You can't Sum calculated fields, you have to repeat the calculation
ie Sum([Credits]-[Debits])