SSRS Sum and iif problem

I have a field "Work_code" which can ent in "0", "1" etc.
I also have a field "Hours". I want to display all the hours if the work_code ends in "0" and sum them.

THe expression
iif(right(Fields!work_code.Value,1)="0",Fields!hours.Value,"")
displays the hours if the work_code end is "0"

But the following does not display the sum - I'm not sure why.
=Sum(iif(right(Fields!work_code.Value,1)="0",Fields!hours.Value,""))