diff --git a/src/Element/CheckboxTree.php b/src/Element/CheckboxTree.php
index 2823a61..fdd3647 100644
--- a/src/Element/CheckboxTree.php
+++ b/src/Element/CheckboxTree.php
@@ -38,7 +38,7 @@ class CheckboxTree extends FormElement {
    */
   public static function processCheckboxes(&$element, FormStateInterface $form_state, &$complete_form) {
     $value = is_array($element['#value']) ? $element['#value'] : [];
-    $allowed = '';
+    $allowed = [];
     if (!empty($element['#max_choices']) && $element['#max_choices'] != '-1') {
       $element['#attached']['drupalSettings'] = [
         'term_reference_tree' => [
@@ -54,7 +54,8 @@ class CheckboxTree extends FormElement {
     if (empty($element['#options'])) {
       $options_tree = [];
       foreach ($element['#vocabularies'] as $vocabulary) {
-        $options_tree += _term_reference_tree_get_term_hierarchy(0, $vocabulary->id(), $allowed, '', '', $value);
+        $options = _term_reference_tree_get_term_hierarchy(0, $vocabulary->id(), $allowed, '', '', $value);
+        $options_tree = array_merge($options_tree, $options);
       }
       $element['#options_tree'] = $options_tree;
       $element['#options'] = _term_reference_tree_get_options($element['#options_tree'], $allowed, NULL);
diff --git a/term_reference_tree.module b/term_reference_tree.module
index aaf27b3..8101eac 100644
--- a/term_reference_tree.module
+++ b/term_reference_tree.module
@@ -136,7 +136,7 @@ function theme_checkbox_tree_label($variables) {
 function _term_reference_tree_get_term_hierarchy($tid, $vid, &$allowed, $filter, $label, $default = array()) {
   $terms = _term_reference_tree_get_children($tid, $vid);
 
-  $result = array();
+  $result = [];
 
   if ($filter != '') {
     foreach ($allowed as $k => $v) {
@@ -208,7 +208,6 @@ function _term_reference_tree_get_children($tid, $vid) {
   while ($term = $results->fetchObject()) {
     $terms[$term->tid] = $term;
   }
-
   return $terms;
 }
 
@@ -221,17 +220,6 @@ function _term_reference_tree_children_selected($terms, $default) {
   return FALSE;
 }
 
-function _term_reference_tree_get_parent($tid) {
-  $q = db_query_range("SELECT h.parent FROM {taxonomy_term_hierarchy} h WHERE h.tid = :tid", 0, 1, array(':tid' => $tid));
-
-  $t = 0;
-  foreach ($q as $term) {
-    $t = $term->parent;
-  }
-
-  return $t;
-}
-
 /**
  * Recursively go through the option tree and return a flat array of
  * options