More than just CurrentMember

[nodoubtom]

I have a calculated member that I'm trying to limit based on a selected member of a dimension.

Here's what I have now:

IIF
(
([Invoice Transaction].CurrentMember IS [Invoice Transaction].[Invoice Transaction].&[1],
([Measures].[Invoice Dollars], [Invoice Transaction].[Invoice Transaction].&[1]) / [Measures].[Invoice Order Count]
, NULL
)

Basically if member 1 is selected as part of the dimension, I want to perform the calculation, otherwise I want to return null.

There are two other members in the dimension that can possibly be selected with the "valid" member for the calcuation, but when I select any of them with the "valid" member, then null is returned.

What I want as a result is to use the calculation if the "valid" member is selected alone or along with other members.

Can this be done?

Thanks in advance!