Undefined index: optional in lib/form/dateselector.php on line 169

developing a module
I wrote in a form:

$mform->addElement('date_selector', $fieldname, get_string($fieldname.'_label', 'collection'), array('startyear' => 1900));

At run time I got the following notice from xdebug:

Undefined index: optional in /Applications/MAMP/htdocs/moodle20/lib/form/dateselector.php on line 169

Actually, in moodle20/lib/form/dateselector.php on line 169 I found

if($arg[2]['optional']) {

and with my call, $arg[2] is:

Array
(
[startyear] => 1900
)

only. Is this a bug?