Can't work out the difference between 2 values.

Hi,

I'm using SQL Server Data Tools to create fetch based reports for my CRM online installation.

In my data query I pull 2 fields that contain values in currency (linetotal and grandtotal).

I'm trying to create an expression against a table column inside my report that simply subtracts the linetotal value from the grandtotal value so I can work out what the discount value is as I sometimes apply a % discount to the linetotal value which is then reflected in the grandtotal value.

I'm using the below code but receive "Error" when I run my report.

=Sum(Fields!grandtotal.Value - Fields!linetotal.Value)

Any help would be appreciated, I'm not that good with SQL!

Many Thanks,

James Bratley