Undefined offset error, form, pdo, php

Question

Notice: Undefined offset: 1 in /var/www/localhost/htdocs/contactdb/add-job-bork.php on line 34

I am getting the above error and data is failing to reach the database. I tried isset but no luck

<?php
$host = "localhost";
$user = "custdb";
$pass = "";
$db = "accounting";
session_start();

$odb = new PDO("mysql:host=" . $host . ";dbname=" . $db, $user, $pass);
$odb->setAttribute(PDO::ATTR_CASE, PDO::ERRMODE_EXCEPTION);
//insert job
if (isset($_POST['invoice_num'])) {
        $cust_id = $_SESSION['id'];
        $date_order = $_POST['date_order'];
        $invoice_num = $_POST['invoice_num'];

        $q = "INSERT INTO orders (cust_id,date_order, invoice_num)
                        SELECT * FROM (SELECT :cust_id,:date_order, invoice_num) as tmp WHERE NOT EXISTS (SELECT invoice_num FROM orders WHERE invoice_num = :invoice_num)
        LIMIT 1;";

        $query = $odb->prepare($q);
        $results = $query->execute(array(
        ":cust_id" => $cust_id,
        ":date_order" => $date_order,
        ":invoice_num" => $invoice_num
        ));
}
?>

<?php
$cust_info = $_SESSION['id'];
$pieces = explode(":", $cust_info);
$_SESSION['id'] = $pieces[0];
if (isset($pieces[1])) {
        $_SESSION['cust_name'] = $pieces[1];
}


?>
        <div class="">
                <form method="post" action"">
                        <label> Order date:</label>
                        <input type="text" id="date_order" name="date_order"/> <br />
                        <input type="text" id="invoice_num" name="invoice_num"/> <br />
                        <input type ="submit" value"Submit" /> <br /> <br />
                </form>
        </div>
</body>