SumIf not giving correct result

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Hi all,

I've been trying to create a simple SumIf expression, which references an expression earlier in the report that I've created to add a small value of 0.50 to invoiced items that come in at 0 value. Here's the referenced expression:

SOPInvoiceCreditLines.LineTotalValue = 0 ? (SOPInvoiceCreditLines.LineTotalValue + 0.50) * SOPOrderReturnLines.LineQuantity : SOPInvoiceCreditLines.LineTotalValue * 1

This works correctly and is fine, but the SumIf I created near the bottom of the report is just looking to get all instances of this same situation.
SumIf(Expr18, SOPInvoiceCreditLines.LineTotalValue=0)
I just can't quite get the values to align correctly, however. Initially it was too high, so I've tried setting the evaluate section to only be the section of this top expression, currently called Expr18, but that's only made it incorrect too low.