diff --git a/core/modules/comment/src/CommentInterface.php b/core/modules/comment/src/CommentInterface.php
index 6cc36850998d..055c875c30bb 100644
--- a/core/modules/comment/src/CommentInterface.php
+++ b/core/modules/comment/src/CommentInterface.php
@@ -211,8 +211,9 @@ interface CommentInterface extends ContentEntityInterface, EntityChangedInterfac
   /**
    * Returns the alphadecimal representation of the comment's place in a thread.
    *
-   * @return string
-   *   The alphadecimal representation of the comment's place in a thread.
+   * @return string|null
+   *   The alphadecimal representation of the comment's place in a thread. NULL
+   *   is returned before a comment is saved.
    */
   public function getThread();
 
diff --git a/core/modules/comment/src/CommentStorageInterface.php b/core/modules/comment/src/CommentStorageInterface.php
index 592b1ea92159..764a5e4a9b95 100644
--- a/core/modules/comment/src/CommentStorageInterface.php
+++ b/core/modules/comment/src/CommentStorageInterface.php
@@ -17,9 +17,10 @@ interface CommentStorageInterface extends ContentEntityStorageInterface {
    * @param \Drupal\comment\CommentInterface $comment
    *   A comment entity.
    *
-   * @return string
+   * @return string|null
    *   The maximum encoded thread value among the top level comments of the
-   *   node $comment belongs to.
+   *   node $comment belongs to. NULL is returned when the commented entity has
+   *   no comments.
    */
   public function getMaxThread(CommentInterface $comment);
 
@@ -29,8 +30,9 @@ interface CommentStorageInterface extends ContentEntityStorageInterface {
    * @param \Drupal\comment\CommentInterface $comment
    *   A comment entity.
    *
-   * @return string
-   *   The maximum encoded thread value among all replies of $comment.
+   * @return string|null
+   *   The maximum encoded thread value among all replies of $comment. NULL is
+   *   returned when the commented entity has no comments.
    */
   public function getMaxThreadPerThread(CommentInterface $comment);
 
diff --git a/core/modules/comment/src/CommentViewBuilder.php b/core/modules/comment/src/CommentViewBuilder.php
index e2da95b850ab..0e4e024dd574 100644
--- a/core/modules/comment/src/CommentViewBuilder.php
+++ b/core/modules/comment/src/CommentViewBuilder.php
@@ -124,7 +124,7 @@ class CommentViewBuilder extends EntityViewBuilder {
 
     foreach ($entities as $id => $entity) {
       if ($build[$id]['#comment_threaded']) {
-        $comment_indent = count(explode('.', $entity->getThread())) - 1;
+        $comment_indent = count(explode('.', (string) $entity->getThread())) - 1;
         if ($comment_indent > $current_indent) {
           // Set 1 to indent this comment from the previous one (its parent).
           // Set only one extra level of indenting even if the difference in
diff --git a/core/modules/comment/src/Entity/Comment.php b/core/modules/comment/src/Entity/Comment.php
index 8da909c50352..2fc615f30808 100644
--- a/core/modules/comment/src/Entity/Comment.php
+++ b/core/modules/comment/src/Entity/Comment.php
@@ -102,7 +102,7 @@ class Comment extends ContentEntityBase implements CommentInterface {
           // by retrieving the maximum thread level.
           $max = $storage->getMaxThread($this);
           // Strip the "/" from the end of the thread.
-          $max = rtrim($max, '/');
+          $max = rtrim((string) $max, '/');
           // We need to get the value at the correct depth.
           $parts = explode('.', $max);
           $n = Number::alphadecimalToInt($parts[0]);