ERROR: invalid input syntax for integer: ""

Following query fails in pgAdmin which is OK because the field PLZZ
contains characters:

select * from "PERSONEN" where (CAST("PERSONEN"."PLZZ" AS INTEGER) >=
70000 );

but if using the same query with a cursor ist works:

START TRANSACTION;

DECLARE c21112234 SCROLL CURSOR FOR select * from "PERSONEN" where
(CAST("PERSONEN"."PLZZ" AS INTEGER) >= 70000;

COMMIT;

Why??
In my opinion this query should also fail with cursors.

I use libpq with PG 8.2.1.
The field PLZZ is of type varchar.