CodeIgniter is_unique Error Message in Language File

When using CodeIgniter I like to set my error messages in application/language/english/form_validation_lang.php which works fine for every error message but does not seem to work for the is_unique message as it gives me the standard message of "The email field must contain a unique value."

My code:

$lang['is_unique'] = "The %s entered is already in use.";