"Field catalog not found" error in ALV program

Hi,

Please help me,I can not find what is error is my code,I have always error message "Field catalog not found" .

report z01.

type-pools: slis .

tables:skat.

data:begin of wa_skat,

        ktopl like skat-ktopl,

        saknr like skat-saknr,

        txt20 like skat-txt20,

      end of wa_skat.

data:i_skat like standard table of wa_skat with header line.

data: fieldcat type slis_t_fieldcat_alv.

select-options:zacc for skat-saknr.

select * into corresponding fields of wa_skat

   from skat

    for all entries in zacc

   where saknr = zacc-low.

   append wa_skat to i_skat.

endselect.

call function 'REUSE_ALV_FIELDCATALOG_MERGE'

   exporting

     i_program_name         = sy-repid

     i_internal_tabname     = 'I_SKAT'

*   I_STRUCTURE_NAME       = 'wa_SKAT'

     i_inclname             = sy-repid

     i_bypassing_buffer     = 'X'

   changing

     ct_fieldcat            = fieldcat

   exceptions

     inconsistent_interface = 1

     program_error          = 2

     others                 = 3.

call function 'REUSE_ALV_GRID_DISPLAY'

   exporting

     it_fieldcat = fieldcat

   tables

     t_outtab    = i_skat.