diff --git a/relation_entity_collector/relation_entity_collector.module b/relation_entity_collector/relation_entity_collector.module
index bc7c512..8848348 100644
--- a/relation_entity_collector/relation_entity_collector.module
+++ b/relation_entity_collector/relation_entity_collector.module
@@ -458,7 +458,11 @@ function relation_entity_collector_views_post_execute($view) {
       $ids = array();
       foreach ($view->result as $row) {
         foreach ($collect as $entity_type => $alias) {
-          $ids[$entity_type][] = $row->$alias;
+          // Skip empty values, which may happen for entities that are obtained
+          // via a non-required relationship in the view.
+          if (!empty($row->$alias)) {
+            $ids[$entity_type][] = $row->$alias;
+          }
         }
       }
       foreach ($ids as $entity_type => $entity_ids) {
diff --git a/tests/relation.views.test b/tests/relation.views.test
index e125bc9..e7241e8 100644
--- a/tests/relation.views.test
+++ b/tests/relation.views.test
@@ -18,7 +18,7 @@ class RelationViewsTestCase extends RelationTestCase {
   }
 
   function setUp() {
-    parent::setUp('relation');
+    parent::setUp('relation', 'relation_entity_collector');
     // While setUp fails for non-existing modules, module_enable() doesn't.
     module_enable(array('views'));
   }