diff --git a/core/modules/comment/lib/Drupal/comment/CommentManager.php b/core/modules/comment/lib/Drupal/comment/CommentManager.php
index 3816fc4e1435..fc833b8efd2c 100644
--- a/core/modules/comment/lib/Drupal/comment/CommentManager.php
+++ b/core/modules/comment/lib/Drupal/comment/CommentManager.php
@@ -56,12 +56,14 @@ class CommentManager implements CommentManagerInterface {
   /**
    * {@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 84a3589e9f62..b1747f2ab8c8 100644
--- a/core/modules/comment/lib/Drupal/comment/CommentManagerInterface.php
+++ b/core/modules/comment/lib/Drupal/comment/CommentManagerInterface.php
@@ -27,7 +27,7 @@ interface CommentManagerInterface {
    * Utility function to return an array of comment fields.
    *
    * @param string $entity_type
-   *   The entity type to return fields which are attached on.
+   *   The content entity type to which the comment fields are attached.
    *
    * @return array
    *   An array of comment field map definitions, keyed by field name. Each
@@ -38,7 +38,7 @@ interface CommentManagerInterface {
    *
    * @see field_info_field_map()
    */
-  public function getFields($entity_type = NULL);
+  public function getFields($entity_type);
 
   /**
    * Utility function to return all comment fields.