diff --git a/core/modules/forum/lib/Drupal/forum/ForumContainerItem.php b/core/modules/forum/lib/Drupal/forum/ForumContainerItem.php
index a2e1da6..ab185ee 100644
--- a/core/modules/forum/lib/Drupal/forum/ForumContainerItem.php
+++ b/core/modules/forum/lib/Drupal/forum/ForumContainerItem.php
@@ -24,7 +24,7 @@ class ForumContainerItem extends BooleanItem {
   static $propertyDefinitions;
 
   /**
-   * Implements \Drupal\Core\TypedData\ComplexDataInterface::getPropertyDefinitions().
+   * {@inheritdoc}
    */
   public function getPropertyDefinitions() {
 
diff --git a/core/modules/forum/lib/Drupal/forum/ForumContainerValue.php b/core/modules/forum/lib/Drupal/forum/ForumContainerValue.php
index a9beda4..2e20b77 100644
--- a/core/modules/forum/lib/Drupal/forum/ForumContainerValue.php
+++ b/core/modules/forum/lib/Drupal/forum/ForumContainerValue.php
@@ -17,7 +17,7 @@
 class ForumContainerValue extends TypedData {
 
   /**
-   * Implements \Drupal\Core\TypedData\TypedDataInterface::getValue().
+   * {@inheritdoc}
    */
   public function getValue() {
     if (!isset($this->value)) {
@@ -27,30 +27,32 @@ public function getValue() {
       $field = $this->parent->getParent();
       $entity = $field->getParent();
       $tid = $entity->tid->target_id;
-      $config = $this->getConfig();
+      // @todo Remove when TypedData objects can receive injected dependencies.
+      //   See https://drupal.org/node/2053415.
+      $config = \Drupal::config('forum.settings');
       $this->value = ($entity->bundle() == $config->get('vocabulary') && in_array($entity->id(), $config->get('containers')));
     }
     return $this->value;
   }
 
   /**
-   * Implements \Drupal\Core\TypedData\TypedDataInterface::setValue().
+   * {@inheritdoc}
    */
   public function setValue($value, $notify = TRUE) {
     if (isset($value)) {
-      throw new ReadOnlyException('Unable to set a computed property.');
+      if (!isset($this->parent)) {
+        throw new InvalidArgumentException('Computed properties require context for computation.');
+      }
+      $field = $this->parent->getParent();
+      $entity = $field->getParent();
+      if (!$entity->isNew()) {
+        throw new ReadOnlyException('Unable to set a computed property.');
+      }
+      else {
+        // The container value can be set for new entities.
+        parent::setValue($value, $notify);
+      }
     }
   }
 
-  /**
-   * Retrieves the forum settings config object.
-   *
-   * @todo Remove when TypedData objects can receive injected dependencies.
-   *   See https://drupal.org/node/2053415.
-   * @return
-   */
-  protected function getConfig() {
-    return \Drupal::config('forum.settings');
-  }
-
 }