getListLabels not working with IF statement

I'm struggling with display of a field of checkboxes, which allow for multiple selection. As intended, if one or more boxes are checked, the record displays and in the case of a multiple selection, the values are separated by a comma.

If there is one or more checkboxes selected, the record displays as intented. My challenge is that if there are no boxes selected, I would like that the record does not display at all.

I'm using the following code:
<?php if ($realty_listingRecord['heating']): ?>
Heating: <?php echo join(', ', getListLabels('realty_listing', 'heating', $realty_listingRecord['heating'])); ?>
<?php endif ?>

Which results in the following error message if (and only if) there are no checkboxes selected:
"Notice: Undefined index: n/a in /vservers/domainname/htdocs/cmsAdmin/lib/viewer_functions.php on line 1162 n/a"

I previously had a selection of "n/a" in the checkbox list, but have since removed it. I suspect that is the "n/a" I see at the end of the error message.

I'm not sure what I need to correct. Any help on this would be greatly appreciated.

Thanks in advance.
Deborah