diff -u b/core/modules/comment/comment.module b/core/modules/comment/comment.module
--- b/core/modules/comment/comment.module
+++ b/core/modules/comment/comment.module
@@ -1265,7 +1265,7 @@
     }
 
     if (!empty($account) && $account->isAuthenticated()) {
-      $comment->setOwnerId($account->id());
+      $comment->setOwner($account);
       $comment->name->value = check_plain($account->getUsername());
     }
     else {
diff -u b/core/modules/comment/comment.tokens.inc b/core/modules/comment/comment.tokens.inc
--- b/core/modules/comment/comment.tokens.inc
+++ b/core/modules/comment/comment.tokens.inc
@@ -148,7 +148,9 @@
           break;
 
         case 'mail':
-          if ($comment->getOwnerId() != 0) {
+          // Check identicality, because getOwnerId() returns NULL if there is no
+          // ID, which evaluates to 0.
+          if ($comment->getOwnerId() !== 0) {
             $mail = $comment->getOwner()->getEmail();
           }
           else {
diff -u b/core/modules/comment/lib/Drupal/comment/CommentFormController.php b/core/modules/comment/lib/Drupal/comment/CommentFormController.php
--- b/core/modules/comment/lib/Drupal/comment/CommentFormController.php
+++ b/core/modules/comment/lib/Drupal/comment/CommentFormController.php
@@ -321,7 +321,7 @@
     // @todo Too fragile. Should be prepared and stored in comment_form()
     // already.
     if (!$comment->is_anonymous && !empty($comment->name->value) && ($account = user_load_by_name($comment->name->value))) {
-      $comment->setOwnerId($account->id());
+      $comment->setOwner($account);
     }
     // If the comment was posted by an anonymous user and no author name was
     // required, use "Anonymous" by default.
diff -u b/core/modules/user/lib/Drupal/user/EntityOwnerInterface.php b/core/modules/user/lib/Drupal/user/EntityOwnerInterface.php
--- b/core/modules/user/lib/Drupal/user/EntityOwnerInterface.php
+++ b/core/modules/user/lib/Drupal/user/EntityOwnerInterface.php
@@ -9,6 +9,11 @@
 
 /**
  * Defines a common interface for entities that have an owner.
+ *
+ * An owner is someone who has primary control over an entity, similar to
+ * owners in Unix file system access. This may or may not be the entity's
+ * original author. The owner may also have less permissions than other users,
+ * such as administrators.
  */
 interface EntityOwnerInterface {