PHPMailer error - Language string failed to load: recipients_failed someone@somewhere.com

Hi. I saw PHPMailer recommened on this site so I'm now trying to use it. However, I'm getting the above error message. All I've done so far is take the example code and changed it to suit my ISP as shown below (Some bits, obviously, changed for secrity reasons - all email addresses being used in my code are real).

Error reads:
Mailer Error: Language string failed to load: recipients_test999@anywhere.com

Code:
<?php

require("class.phpmailer.php");

$mail = new PHPMailer();

$mail->IsSMTP();                                      // set mailer to use SMTP
$mail->Host = "smtp.mydomain.com"; // specify main and backup server
$mail->SMTPAuth = true;     // turn on SMTP authentication
$mail->Username = "myname";  // SMTP username
$mail->Password = "mypass"; // SMTP password

$mail->From = "test@legitimateaddress.com";
$mail->FromName = "This is my name";
$mail->AddAddress("test999@anywhere.com", "YourName");
$mail->AddReplyTo("test@legitimateaddress.com", "Test");

$mail->WordWrap = 50;                                 // set word wrap to 50 characters
$mail->IsHTML(true);                                  // set email format to HTML

$mail->Subject = "This is a test message";
$mail->Body    = "This is the HTML message body <b>in bold!</b>";
$mail->AltBody = "This is the body in plain text for non-HTML mail clients";

if(!$mail->Send())
{
   echo "Message could not be sent. <p>";
   echo "Mailer Error: " . $mail->ErrorInfo;
   exit;
}

echo "Message has been sent";
?>

Thanks in advance for help and advice :)
DC