diff --git a/core/modules/comment/lib/Drupal/comment/CommentManager.php b/core/modules/comment/lib/Drupal/comment/CommentManager.php
index 3816fc4..fc833b8 100644
--- a/core/modules/comment/lib/Drupal/comment/CommentManager.php
+++ b/core/modules/comment/lib/Drupal/comment/CommentManager.php
@@ -56,12 +56,14 @@ public function getParentEntityUri(CommentInterface $comment) {
   /**
    * {@inheritdoc}
    */
-  public function getFields($entity_type = NULL) {
-    $map = $this->getAllFields();
-    if (!isset($map[$entity_type])) {
+  public function getFields($entity_type) {
+    $info = $this->entityManager->getDefinition($entity_type);
+    if (!is_subclass_of($info['class'], '\Drupal\Core\Entity\ContentEntityInterface')) {
       return array();
     }
-    return $map[$entity_type];
+
+    $map = $this->getAllFields();
+    return isset($map[$entity_type]) ? $map[$entity_type] : array();
   }
 
   /**
diff --git a/core/modules/comment/lib/Drupal/comment/CommentManagerInterface.php b/core/modules/comment/lib/Drupal/comment/CommentManagerInterface.php
index 84a3589..1f091d9 100644
--- a/core/modules/comment/lib/Drupal/comment/CommentManagerInterface.php
+++ b/core/modules/comment/lib/Drupal/comment/CommentManagerInterface.php
@@ -38,7 +38,7 @@ public function getParentEntityUri(CommentInterface $comment);
    *
    * @see field_info_field_map()
    */
-  public function getFields($entity_type = NULL);
+  public function getFields($entity_type);
 
   /**
    * Utility function to return all comment fields.