SSRS #Error when trying to sum on an expersion field

I have a report where it is returning #Error when I try to sum a field at the group level.  I have added a calculated column called BTD  ...  =iif(Fields!Etime_Actuals.Value = 0,0,Fields!Etime_Budget.Value)

On the Details line the values are returned as expected  (In my example there are 4 rows, 2 with values and 2 with 0)

When I try and insert at the Group level (only one group) Sum(BTD) and run the report it returns #Error for this cell. 

Any ideas on what I am doing wrong here?

I am doing this in Report Builder 2.0  (SQL Server 2008 R2)



Andrew Payze