Mandrill ValidationError

I am getting the following error when trying to send a mail through Mandrill's API:

{"status":"error","code":-1,"name":"ValidationError","message":"You must specify a key value"}

The code that follows is what I am using to try to send the mail:

<?php
$to = 'their@email.com';
$content = '<p>this is the emails html <a href="www.google.co.uk">content</a></p>';
$subject = 'this is the subject';
$from = 'my@email.com';

$uri = 'https://mandrillapp.com/api/1.0/messages/send.json';
$content_text = strip_tags($content);

$postString = '{
"key": "RR_3yTMxxxxxxxx_Pa7gQ",
"message": { 
 "html": "' . $content . '",
 "text": "' . $content_text . '",
 "subject": "' . $subject . '",
 "from_email": "' . $from . '",
 "from_name": "' . $from . '",
 "to": [
 {
 "email": "' . $to . '",
 "name": "' . $to . '"
 }
 ],
 "track_opens": true,
 "track_clicks": true,
 "auto_text": true,
 "url_strip_qs": true,
 "preserve_recipients": true
},
"async": false
}';

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $uri);
curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true );
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true );
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_POSTFIELDS, $postString);
$result = curl_exec($ch);
echo $result;

?>

What could be causing the validation error in the message. I am providing my API key, AND it's valid!

Hope someone will be able to help, and thanks for generally being AWESOME here!

Thanks!

You may also want to just use arrays, and let PHP handle the JSON encoding for you. This particular error is common if the JSON is invalid for some reason. So, for example, you could set your parameters like this:

$params = array(
    "key" => "keyhere",
    "message" => array(
        "html" => $content,
        "text" => $content_text,
        "to" => array(
            array("name" => $to, "email" => $to)
        ),
        "from_email" => $from,
        "from_name" => $from,
        "subject" => $subject,
        "track_opens" => true,
        "track_clicks" => true
    ),
    "async" => false
);

$postString = json_encode($params);

You can also use json_decode to parse the response if needed.