Invalid argument supplied for foreach()

I haven't seen this reported, but upon every cron run I see the following message in my Drupal log:

Invalid argument supplied for foreach() in /var/www/html/gochicorgohome.com/sites/all/modules/similarity/similarity.module on line 479.

Here's the code at that point:

/** 
 * Implementation of hook_cron().
 */
function similarity_cron() {
  $all = similarity_all_similarities(TRUE);
  foreach($all as $sim_obj) {
    $sim_obj->calculate();
  }
}