"Notice: undefined property" warning

I get this after using the_relative_date():

Notice: Undefined property: ia_Relative_Date::$_instance in /Users/ian/Sites/wordpress/wp-content/plugins/relative-date-template-tag/assets/classes/iaRelativeDate.class.php on line 24

If I change line 24 to use isset(), the notice goes away:

if(isset($this->_instance)) {

Great plugin, thanks!

http://wordpress.org/extend/plugins/relative-date-template-tag/