Do I use the ID transformation in a right way?

I experience a confusing issue with built-in SAP transformation ID.

I try to serialize ABAP structure, but result XML is always empty. Do you have any suggestions, what is wrong with my code?

DATA lv_xml TYPE xstring.

CALL TRANSFORMATION ID
  SOURCE test = syst
  RESULT XML = lv_xml.

IF lv_xml IS INITIAL.
  MESSAGE 'Oops, it's empty!' TYPE 'S' DISPLAY LIKE 'W'.
ELSE.
  CALL FUNCTION 'DISPLAY_XML_STRING'
    EXPORTING
      xml_string = lv_xml.
ENDIF.