Work Area 'ITAB' is not long enough ?

Hello Gurus,

i have an internal table ITAB, which has all the fields of DB table ZDESC which has all total 6 fields ( 3-key fields).

I am fetching 4 fields from scrren to ITAB including the key fields.

and then saving these data into ZDESC table .

But here the error is - work area 'ITAb' is not long enough.

whats the reason and how to solve this ?

here is my code , its so simple

.

PAI include.

-

case ok_code.

when 'enter'

  IT_HEAD-CCODE = S_CCODE .                                   * s_code = screen field.
  IT_HEAD-PCODE = S_PCODE .                                    *  first three field are key fields except c_name
   IT_HEAD-BCODE = S_BCODE .
  IT_HEAD-CNAME = S_CNAME .  

when 'save'.

modify zdesc from table it_head.

endcase.

dats it.