diff --git a/src/Element/CheckboxTree.php b/src/Element/CheckboxTree.php
index e822e6c..e0f222e 100644
--- a/src/Element/CheckboxTree.php
+++ b/src/Element/CheckboxTree.php
@@ -24,11 +24,13 @@ class CheckboxTree extends FormElement {
       '#input' => TRUE,
       '#tree' => TRUE,
       '#process' => [
-        [$class, 'processCheckboxes'],
+        [$class, 'processCheckboxTree'],
       ],
       '#pre_render' => [
         [$class, 'preRenderCompositeFormElement'],
       ],
+      '#default_value' => [],
+      '#attached' => ['library' => ['term_reference_tree/term_reference_tree']],
       '#theme' => 'checkbox_tree',
     ];
   }
@@ -36,7 +38,7 @@ class CheckboxTree extends FormElement {
   /**
    * Processes a checkboxes form element.
    */
-  public static function processCheckboxes(&$element, FormStateInterface $form_state, &$complete_form) {
+  public static function processCheckboxTree(&$element, FormStateInterface $form_state, &$complete_form) {
     $value = is_array($element['#value']) ? $element['#value'] : [];
     $allowed = [];
     if (!empty($element['#max_choices']) && $element['#max_choices'] != '-1') {
@@ -65,8 +67,8 @@ class CheckboxTree extends FormElement {
 
     $tree = new \stdClass();
     $tree->children = $terms;
+    unset($element['#needs_validation']);
     $element[] = _term_reference_tree_build_level($element, $tree, $form_state, $value, $element['#max_choices'], [], 1);
-
     return $element;
   }
 
@@ -74,25 +76,12 @@ class CheckboxTree extends FormElement {
    * {@inheritdoc}
    */
   public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
-    if ($input === FALSE) {
-      $value = [];
-      $element += ['#default_value' => []];
-      foreach ($element['#default_value'] as $key) {
-        $value[$key] = $key;
-      }
-      return $value;
-    }
-    elseif (is_array($input)) {
-      foreach ($input as $key => $value) {
-        if (!isset($value)) {
-          unset($input[$key]);
-        }
-      }
-      return array_combine($input, $input);
-    }
-    else {
-      return [];
+    $value = [];
+    $element += ['#default_value' => []];
+    foreach ($element['#default_value'] as $key) {
+      $value[$key['target_id']] = $key['target_id'];
     }
+    return $value;
   }
 
 }
diff --git a/src/Plugin/Field/FieldWidget/TermReferenceTree.php b/src/Plugin/Field/FieldWidget/TermReferenceTree.php
index a4984ed..c02181c 100644
--- a/src/Plugin/Field/FieldWidget/TermReferenceTree.php
+++ b/src/Plugin/Field/FieldWidget/TermReferenceTree.php
@@ -41,19 +41,55 @@ class TermReferenceTree extends WidgetBase {
     $handler_settings = $this->getFieldSetting('handler_settings');
     $vocabularies = Vocabulary::loadMultiple($handler_settings['target_bundles']);
 
-    if (!array_key_exists('#value', $element)) {
-      $element['#value'] = array();
-    }
-
-    $element['#attached']['library'][] = 'term_reference_tree/term_reference_tree';
     $element['#type'] = 'checkbox_tree';
-    $element['#default_value'] = (is_object($items[$delta])) ? $items[$delta]->value : [];
+    $element['#default_value'] = $items->getValue();
     $element['#vocabularies'] = $vocabularies;
     $element['#max_choices'] = $this->fieldDefinition->getFieldStorageDefinition()->getCardinality();
     $element['#leaves_only'] = FALSE;
+    $element['#value_key'] = 'target_id';
     $element['#max_depth'] = 0;
     $element['#start_minimized'] = TRUE;
+    $element['#element_validate'] = [[get_class($this), 'validateTermReferenceTreeElement']];
     return $element;
   }
 
+  /**
+   * Form element validation handler for term reference form widget.
+   */
+  public static function validateTermReferenceTreeElement(&$element, FormStateInterface $form_state) {
+    $items = _term_reference_tree_flatten($element, $form_state);
+    $value = [];
+    if ($element['#max_choices'] != 1) {
+      foreach ($items as $child) {
+        if (!empty($child['#value'])) {
+          array_push($value, array($element['#value_key'] => $child['#value']));
+
+          // If the element is leaves only and select parents is on, then automatically
+          // add all the parents of each selected value.
+          if ($element['#select_parents'] && $element['#leaves_only']) {
+            foreach ($child['#parent_values'] as $parent_tid) {
+              if (!in_array(array($element['#value_key'] => $parent_tid), $value)) {
+                array_push($value, array($element['#value_key'] => $parent_tid));
+              }
+            }
+          }
+        }
+      }
+    }
+    else {
+      // If it's a tree of radio buttons, they all have the same value, so we can just
+      // grab the value of the first one.
+      if (count($items) > 0) {
+        $child = reset($items);
+        if (!empty($child['#value'])) {
+          array_push($value, array($element['#value_key'] => $child['#value']));
+        }
+      }
+    }
+    if ($element['#required'] && empty($value)) {
+      $form_state->setError($element, t('%name field is required.', array('%name' => $element['#title'])));
+    }
+    $form_state->setValueForElement($element, $value);
+  }
+
 }