PDO SQL-state "00000" but still error?

Can anybody explain why

$sql->execute($params);

returns FALSE, whereas

print $pdo->errorCode();
print_r($pdo->errorInfo());

both return SQLSTATE 00000, which means according to the documentation success? It is an INSERT and nothing is actually being inserted into the database... so, why do I get a success message from SQLSTATE?

In case it helps, this is the code...

$sql = $pdo->prepare("
        INSERT INTO user (
            username, fname, pass, salt, email,
            loc_id_home, country_id_home, region_id_home,
            cont_id_home, timestamp_reg, timestamp_upd, timestamp_lastonline, 
            online_status, gender, birthdate
            )
        VALUES (
            :username,:fname,:pass,:random_salt,:email,
            :loc_id_home,:country_id_home,:region_id_home,
            :cont_id_home,'".time()."','".time()."','".time()."',
            1,:gender,:birthdate)
        ");

$params=array(
    ':username'=>$username,
    ':fname'=>$fname,
    ':pass'=>$pass,
    ':random_salt'=>$random_salt,
    ':email'=>$email,
    ':loc_id_home'=>$loc_id_home,
    ':country_id_home'=>$country,
    ':region_id_home'=>$region,
    ':cont_id_home'=>$continent,
    ':gender'=>$gender,
    ':birthdate'=>$birthdate
);  

$sql->execute($params);

print $pdo->errorCode();
print_r($pdo->errorInfo());