EPMSelectMember - #Ambiguous MemberID

Hi

I'm trying to write EPMSelectMember Formula

My Hirearchy is

BIG                                TOP LEVEL

  APAC                            LEVEL 1

      APAC_INPUT            BAS MEMBER

            AU-NZ                 LEVEL2

                AUSTRALIA     LEVEL3

                       AU             BASMEMBER 

                 NEWZELAND   LEVEL3

                          NZ           BAS MEMBER   

     BRUNEI

           BRUNEI-SR

                 BN

     CAMBODIA

etc.,

when i write formula EPMSelectMember(,"APAC","PARENTH1=BIG","COUNTRY",FALSE) , error says #Ambiguous MemberID

but when i write formula EPMSelectMember(,"APAC_INPUT","PARENTH1=BIG","COUNTRY",FALSE)  no error

but again when i write formula EPMSelectMember(,"AU","PARENTH1=BIG","COUNTRY",FALSE) again error says #Ambiguous MemberID

I didn't understand limitations of EPMSelectMember

please find attached error image, and please let me know what are the members do i need to use in EPMSelectMember.

Regards

Silu