PHP notice undefined variable "quillUnits" when using proximity search with "km" units

I created a simple proximity-search-enabled profile, then used that profile form to search for contacts (restricted to a group, but I don't think that's part of the issue). When I select "km" for distance units, I get the following notice on the results page (same notice repeated twice):

Notice: Undefined variable: qillUnits in CRM_Contact_BAO_ProximityQuery::process() (line 327 of /Applications/MAMP/htdocs/mysite/sites/all/modules/civicrm/CRM/Contact/BAO/ProximityQuery.php).

I get the same notice (only once) if I click 'map these contacts' on the results list.

The results list appears to be correct - if I edit the search criteria units to "miles" I get more results - and no PHP notices.

I couldn't reproduce on the (Drupal 7) demo sandbox site, probably because warnings and notices are being suppressed.

The solution seems to be to change line 327 of ProximityQuery.php to use "=" instead of ".=" in the "else" branch. When I do so, all notices go away. I just want to confirm that there's another reason for the concatenation operator in this instance that I haven't fathomed. the main branch of the same if..then..else uses the straight assignment operator in the case of "miles", so it appears that there's nothing to concatenate onto in the "else" branch.