Error Counting Dimension Level in Calculated Member

[gbim]

I have a calculated member with the following expression:

iif(Count(Descendants([Cp Date].CurrentMember, [Cp Week]))<>0, [Measures].[Ocp Css] / Count(Descendants([Cp Date].CurrentMember, [Cp Week]),INCLUDEEMPTY),0)

Basicaly this have to make a division between the measure "Ocp Css" with the count of the weeks (Cp Week).

The problem is when I set a filter in 2 years in Cp Date
Dimension. All the data is blank in this situation.
When there is not any filter or only one year is selected everything works fine.

Anyone can help me ?
Thank you,