ABAP Dump TABLE_INVALID_INDEX

Hi

Urgent....

Am getting dump on below code after system upgrade please suggest suitable SNOTE.

LOOP AT it_xvbap.

     IF it_xvbap-matnr CO ' 0123456789'.

       CALL FUNCTION 'CONVERSION_EXIT_MATN1_INPUT'

         EXPORTING

           input        = it_xvbap-matnr

         IMPORTING

           output       = it_xvbap-matnr

         EXCEPTIONS

           length_error = 1

           OTHERS       = 2.

       MODIFY it_xvbap INDEX sy-tabix FROM it_xvbap TRANSPORTING matnr .

       CLEAR it_xvbap.

     ENDIF.

   ENDLOOP.

In debugging i found that sy-tabix  is 0 and sy-index is 0.but it_xvbap table have one record

Its is happening only after the upgrade only