Wow, so iserror([Date Dimension].[Fiscal Year Hierarchy].CurrentMember) does not test whether the CurrentMember function returns an error? MDX is still very counter-intuitive to me in many ways.

Thanks for offering the alternative formulation. Over the past few days I have re-arranged measure definitions so that the need to check for more-than-one date range is no longer necessary. So the problem has (well, seems to have) gone away.

--Matt.