Need to sum the difference of two summary columns

[Jack_Bates]

I can sum the two summary columns in the reports

However I need to omit from the total the data from the column when the other column is empty

In Table “Basic Summary” 

Fuel Amount: SUM(SELECT([Expenses][AMOUNT], [EXPENSE] = "Fuel"))

Fuel Discount: SUM(SELECT(Expenses[Fuel Discount], AND([DATE] >= [_THISROW].[start date], [DATE] <= [_THISROW].[end date])))

[Fuel Amount]-[Fuel Discount]

I need to sum this but exclude the data from “Fuel amount” where the “Fuel Discount is missing

Basic Summary TableExpenses Table