LSMW: Ensure field sequence is correct for data for multiple source structures.

Hi,

I want to carry some changes to the PO (ME22N) via a  LSMW (BAPI). A simplified example of the source fields:

  HEADERDATA

       IDENTIFIER: H

       EBELN: po number

                              ITEMDATA (lower level)

                                     IDENTIFIER:  I

                                     EBELP: item number

                                     ELIKZ: delivery completed

When the system try to read the file, there is an error message: Ensure field sequence is correct for data for multiple source structures.

  Message no. /SAPDMC/LSMW_OBJ_060026

Please provide me how to structure the file using the identifiers and the fields to upload the data via LSMW ?

Thanks