Ik krijg de volgende melding: undefined index: subject in public/sites/www.protree.nl/php/contact.ph

Bovenstaande melding krijg ik na het invullen en versturen van het contactformulier. Nu ben ik een beginner wat php betreft en weet niet hoe ik dit moet oplossen. Onderstaand het contactformulier.
Kunnen jullie mij helpen. Alvast bedankt.

<?php
include('contact-config.php');
$contact_form_version=strtolower($contact_form_version);
if(empty($my_email_address) || empty($contact_form_version) || ($contact_form_version!='php' && $contact_form_version!='html')) {
die('Error: The contact form has not been setup correctly.');
} else {
if($contact_form_version=='php') {
$formLocation='../contact.php';
} else if($contact_form_version=='html') {
$formLocation='../contact.html';
}
}
if(!empty($_POST['ajax']) && $_POST['ajax']=='1') {
$ajax=true;
} else {
$ajax=false;
}
if(empty($_POST['name']) || empty($_POST['email']) || empty($_POST['message'])) {
if($ajax || $contact_form_version=='html') {
die('U heeft niet alles/goed ingevuld.');
} else {
header("Location: $formLocation?msg=missing-variables");
exit;
}
}
if (!preg_match("/^([A-Za-z0-9_\-\.])+\@([A-Za-z0-9_\-\.])+\.([A-Za-z]{2,8})$/", $_POST['email'])) {
if($ajax || $contact_form_version=='html') {
die('Error: Invalid email address');
} else {
header("Location: $formLocation?msg=invalid-email-address");
exit;
}
}
$name=$_POST['name'];
$email=$_POST['email'];
$subject=$_POST['subject'];
$message=$_POST['message'];
$to=$my_email_address;
$headers = 'From: '.$name.' <'.$email.'>'." \r\n";
$headers .= 'Reply-To: '.$email."\r\n";
if(isset($cc) && $cc!='') {
$headers .= 'Cc: '.$cc."\r\n";
}
if(isset($bcc) && $bcc!='') {
$headers .= 'Bcc: '.$bcc."\r\n";
}
$headers .= 'X-Mailer: PHP/' . phpversion();
$subject = $subject;
$message = wordwrap($message, 70);
if(mail($to, $subject, $message, $headers)) {
if($ajax || $contact_form_version=='html') {
die('De email is verstuurd');
} else {
header("Location: $formLocation?msg=mail-sent");
exit;
}
} else {
if($ajax || $contact_form_version=='html') {
die('Error: Mail failed');
} else {
header("Location: $formLocation?msg=mail-failed");
exit;
}
}
?>