Isfiltered is not working on date hierarchy

Hi guys,

I've got a problem here with IsFiltered function. I've already used it several times, but not with dates hierarchy.

For example, I created a measure like this:

Previous Sum = if(ISFILTERED(Dataset[RefDate].[Year]);
		CALCULATE(SUM(Dataset[Values]);PREVIOUSMONTH(Dataset[RefDate].[Date]));
		CALCULATE(SUM(Dataset[Values]);PREVIOUSYEAR(Dataset[RefDate].[Date])))

I use this on a bars graph with drill down on Year/Month. So, when I'm visualizing the year layer, I should have the sum of last year values for each bar. When I drill down to see months, I should have the sum of the last month for each bar. What i'm getting is always the sum of the last month, even on the Year layer.

Any help, please?