--- user_reference/user_reference.module	2011-02-02 14:59:51.000000000 +0100
+++ user_reference/user_reference.module	2011-02-02 15:00:35.000000000 +0100
@@ -480,6 +480,9 @@
   elseif ($ids) {
     $query->condition($user_uid_alias, $ids, 'IN', $ids);
   }
+  else {
+    $query->condition('u.uid', 0, '>');
+  }
 
   $query
     ->orderBy($user_name_alias);