Conditional statement with dimensional model source

[HERRERA]

Hi !

Can i use Case statement at dimensional cube data source ? Im trying use this :

Case
When [fieldX] > 0
Then [fieldX]
Else [fieldY]
End

The expression its correct, dont have error, but when run the report, the following message appears :

The item '___ns_0' is not accessible.

I would like to know if you can use this type of expression in cubes.

hugs,
Herrera