Total On A Conditional Field

I have to 2 texboxes in the detail line of a table

TextBoxA = IIF( Fields!Total_Amount.Value >0, Fields!Total_Amount.Value,0)

TextBoxB =IIF( Fields!Total_Amount.Value <0, Fields!Total_Amount.Value,0)

I would like to total thoses textboxes in the group footer (Loan_Group1)

I tried this for

=Sum(IIF( Fields!Total_Amount.Value > 0, Fields!Total_Amount.Value,0))

Or

=RunningValue(

IIF( Fields!Total_Amount.Value >0, Fields!Total_Amount.Value,0) ,sum,"Loan_Group1")

for both I get this error:

The value expression for the textbox €˜textbox190€™ uses an aggregate function on data of varying data types. Aggregate functions other than First, Last, Previous, Count, and CountDistinct can only aggregate data of a single data type.

Thanks

Elias