flat assembler - struct macro - problem with fields

I have a structure:

Code:

struct IRP

        fwType                  dw ?
        cbSize                  dw ?

        MdlAddress              dd ?
        Flags                   dd ?
        union
                AssociatedIrp.MasterIrp         dd ?
                AssociatedIrp.IrpCount          dd ?
                AssociatedIrp.SystemBuffer      dd ?
        ends
        ThreadListEntry         LIST_ENTRY
        IoStatus                IO_STATUS_BLOCK
        RequestorMode           db ?
        PendingReturned         db ?
        StackCount              db ?
        CurrentLocation         db ?
        Cancel                  db ?
        CancelIrql              db ?
        ApcEnvironment          db ?
        AllocationFlags         db ?
        UserIosb                dd ?
        UserEvent               dd ?
                union
                struct
                        Overlay.AsynchronousParameters.UserApcRoutine  dd  ?
                        Overlay.AsynchronousParameters.UserApcContext  dd  ?
                ends
                Overlay.AllocationSize  dq ?
        ends
        CancelRoutine           dd ?
        UserBuffer              dd ?
        union
                struct
                union
                        Tail.Overlay.DeviceQueueEntry   KDEVICE_QUEUE_ENTRY
                        Tail.Overlay.DriverContext      rd 4
                ends
                Tail.Overlay.Thread                     dd ?
                Tail.Overlay.AuxiliaryBuffer            dd ?
                struct
                        Tail.ListEntry       LIST_ENTRY
                        union
                                Tail.Overlay.CurrentStackLocation       dd ?
                                Tail.Overlay.PacketType                 dd ?
                        ends
                ends
                Tail.Overlay.OriginalFileObject         dd ?
                ends
        Tail.Apc                KAPC
        Tail.CompletionKey      dd  ?
        ends

ends

...and this code woks fine:

Code:

        virtual at ebx
        pIrp    IRP
        end virtual

        mov     eax, [pIrp.Tail.Overlay.CurrentStackLocation]

but that one generates an error /'Error: Undefined symbol'/

Code:

        mov     eax, [ebx+IRP.Tail.Overlay.CurrentStackLocation]

What is wrong?

ps: sorry for my english