SAMEPERIODLASTYEAR returns Blank

I have data that looks like this:

For a selected Cut-off Date, I want to show the sum of sales for both the selected date and the date one year prior. My issue now is that I can't just use SAMEPERIODLASTYEAR on the Cut-off Date, since the exact day changes from year to year (in 2018, it's the 23rd; in 2019, it's the 24th). I figured I could just add an auxiliary column that contains the first of each month rather than the actual cut-off date (see column Aux_Date). I built two simple measures:

Sum_Sales = SUM(Data[Sales])

and

Sum_Sales_LY = CALCULATE([Sum_Sales], SAMEPERIODLASTYEAR(Data[Aux_Date])).

But when I put these on my dashboard, Sum_Sales_LY will show (Blank), if I filter on Cut-off Date:

It works fine when I filter on Aux_Date, but that's not what I want:

Is there any way to utilize SAMEPERIODLASTYEAR without having to switch to Aux_Date as filter?

Maybe I'm struggling because I can't quite wrap my head around as to why SAMEPERIODLASTYEAR is a valid input for the <filter> argument of the CALCULATE formula. The documentation suggests that SAMERPERIODLASTYEAR returns a single column of data (i.e. the dates incremented by -1 year), and I don't understand how that serves as a filter to anything.