ERROR: malformed record literal: "",DETAIL: Missing left parenthesis?

Good morning,

A question about: ERROR:  malformed record literal: ""
DETAIL:  Missing left parenthesis.

Can someone tell me what cause the error?

========================
Table z_drop;
    Column    |          Type
-------------+------------------------
  run_date    | character varying(128)
  adm_year    | character varying(4)
  adm_sess    | character varying(1)
  faculty     | character varying(128)
  ac_cycle    | character varying(128)
  deg_code    | character varying(128)
  discipline  | character varying(128)
  thesis      | character varying(128)
  elig_stype  | character varying(128)
  stud_source | character varying(128)
  applied     | numeric
  reviewed    | numeric
  accepted    | numeric
  confirmed   | numeric
  registered  | numeric
  hold        | numeric
  forward     | numeric
  refused     | numeric
  cancelled   | numeric
  other       | numeric
  pending     | numeric


PREPARE test(z_drop) AS  INSERT INTO z_drop VALUES  ($1, $2, $3, $4, $5,
$6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16, $17, $18, $19, $20,
$21) ;



EXECUTE test('', '1', '1', '1', '1', '1', '1', '1', '1', '', 1, 1, '1',
'0', '0', '0', '0', '0', '0', '0', '0') ;




Thank you,
Emi