SQL SSRS Sum Error

This happens because data is returning nulls or some other bad data that cannot be summed.
Wrap all values in Cdbl(Salesamt) to convert it to a proper decimal


Sum((IIf(Fields!Capacity.Value Is Nothing, CDbl(0),Fields!Capacity.Value))*(IIf(Fields!EstLF.Value Is Nothing, CDbl(0),Fields!EstLF.Value))*8760)"