Trying to create measure to calculate for 'Sameperiodlastyear'

Hi,
I'm following a tutorial, where we're calculating the % difference between years, standard formula; Current period - Previous period / Previous period.
But as we're in a data model using DAX functions;

CALCULATE(Total sales measure, SAMEPERIODLASTYEAR(dDate[Year]))

dDate is the date table created from the sales table. I continually get an error message and can't get the 'alert' to show what the error is.

I've attached the workbook, and the measure ' previous' can be seen in data model fSales table.
The pivot table for which the measure is to be used is on sheet 1 .


Richard.