MDX Operation, Syntax correct, Values = NULL

I Need to do the following calculation in mdx (i hope the code is straightforward, if clarification is needed please tell)

([Orders].[Customer].CurrentMember, [Measures].[InvoiceAmount]) / (Root([Orders]), [Measures].[InvoiceAmount])

but all I'm getting is NULL Values, can anybody see what is Wrong ? Thanks to all of you in advance, your always great help is much appreciated