Creating a sum in SSRS for a calculated field

I am creating what I thought was a fairly simple report in SSRS.  One field, in the detail section is a calculated field.  The formula is as follows:  =IIf(Fields!Wbillable.Value = 1, Fields!Whours.Value,0).  That calculated field works fine in the detail section. however, I am unable to add a total to the group.  I get error#.  And if I right click the field the "Add Total" option is grayed out.   If I simply add the field Whours to the report, that totals fine, so I know it's a numeric field type.   I have tried adding CInt at the beginning and enclosing the IIF statement in () - but that did not help.  I also tried adding CInt in front of each part of the statement - but that didn't help either.  Any assistance here would be appreciated!!  Thank you.
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Tax / Financial Software

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