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    |
charactervarying(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