Issue members loop Birthday

Hi
I´m using your cool plugin! Thank you very much for your great work!
I ´ve add the following code to my members loop:

<?php echo xprofile_get_field_data( 'Geburtsdatum', $theUserID );?>

Everything works fine, but for every member which has not filled out the field ‘Geburtsdatum’ (German for birthday 🙂 ) the members-loop set the actual date of today. How can I change this, that it returns nothing?

Thank you
Cheers
Marco

http://wordpress.org/plugins/buddypress-xprofile-custom-fields-type/