Error: Internal Tables, Field symbol has not yet been assigned

Hi all friend, I am try understand this error: Field symbol has not yet been assigned

I get a dump after run this abap program.

REPORT ZINTERNALTABLES.

tables: bsis.

types: begin of it_bsis_line,

  zuonr like bsis-zuonr,

  belnr like bsis-belnr,

end of it_bsis_line.

data: it_bsis type table of it_bsis_line, lv_i type i.

field-symbols <fst_bsis_line> type any table.

field-symbols <fs> like line of it_bsis.

clear <fst_bsis_line>.

lv_i = 1.

select * up to 10 rows from bsis into table <fst_bsis_line> where zuonr ne ''.

loop at <fst_bsis_line> assigning <fs>.

  write:/ lv_i,' - ', <fs>-zuonr, ' - ', <fs>-belnr.

  lv_i = lv_i + 1.

endloop.

the problem is in this line:

   17 field-symbols <fs> like line of it_bsis.

>>>>> clear <fst_bsis_line>.

   19 lv_i = 1.

any advice what I do wrong?

thanks