• Allthough the offered solution in the post isn't excactly what you need, you are pointed to the use of "IsNothing". So implementing this in your code it will be something like:

    =iif(IsNothing(SUM(Fields!Performance_Amount_Usd.Value)),0,SUM(Fields!Performance_Amount_Usd.Value))

    ** Don't mistake the ‘stupidity of the crowd’ for the ‘wisdom of the group’! **