diff --git a/taxonomy_manager.admin.inc b/taxonomy_manager.admin.inc
index 50c934de99f23c2ebf16c5119d6e8a365b1a28b4..6781a305f3f54e7e935292585e98fa93bff6c9f5 100644
--- a/taxonomy_manager.admin.inc
+++ b/taxonomy_manager.admin.inc
@@ -325,7 +325,7 @@ function taxonomy_manager_double_tree_form($form, &$form_state, $voc1, $voc2, $t
     $tids_to_expand = $form_state['values']['double_tree_values']['right_tree_expand_terms'];
   }
   else {
-    $tids_to_expand = $tid;
+    $tids_to_expand = $tid ? [$tid] : [];
   }
 
   $form['taxonomy2']['manager']['tree'] = [