MDX to always return the same number of columns

I have a query which has a WHERE clause in my MDX statement which when applied can restrict the set members (and therefore columns) that are returned. I need the number of columns returned to be the same every time. Is there a simple way to always return all members of the set regardless of the WHERE clause?This seems to be to do with the set up of my cube. I don't understand why. I have a customer flag attribute on my customer dimension. If I use this in my WHERE clause NON EMPTY columns aren't returned. I also have an order flag dimension where order flag is the only attribute. If this is used in the WHERE then they are. The only difference that I can see is that order flag is the key column of the dimension. I don't know whether that's the reason for this behaviour?