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