I've got a fairly complex formula in SSRS and for some reason it shows an error when it encounters a zero value, even though I'm handling zeros in my formula. Can anybody advise how to fix this issue?

=iif(datepart("h",Now())=6, iif(SUM(Fields!PYRevenue.Value) = 0, 0,((SUM(Fields!Revenue.Value) - SUM(Fields!PYRevenue.Value)) / SUM(Fields!PYRevenue.Value)))
,iif(SUM(Fields!PYRevenue.Value) = 0, 0,(SUM(Fields!Revenue.Value) / SUM(Fields!PYRevenue.Value))))