core/modules/comment/src/CommentViewBuilder.php                         | 2 +-
 core/modules/comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/core/modules/comment/src/CommentViewBuilder.php b/core/modules/comment/src/CommentViewBuilder.php
index 0376a77..fe27860 100644
--- a/core/modules/comment/src/CommentViewBuilder.php
+++ b/core/modules/comment/src/CommentViewBuilder.php
@@ -156,7 +156,7 @@ public function buildComponents(array &$build, array $entities, array $displays,
 
       $account = comment_prepare_author($entity);
       $config = \Drupal::config('user.settings');
-      $build['#cache']['tags'] = Cache::mergeTags($build['#cache']['tags'],  $config->getCacheTags());
+      $build['#cache']['tags'] = Cache::mergeTags(isset($build['#cache']['tags']) ? $build['#cache']['tags'] : [],  $config->getCacheTags());
       if ($config->get('signatures') && $account->getSignature()) {
         $build[$id]['signature'] = array(
           '#type' => 'processed_text',
diff --git a/core/modules/comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php b/core/modules/comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php
index 1a97e1b..fc4501d 100644
--- a/core/modules/comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php
+++ b/core/modules/comment/src/Tests/CommentDefaultFormatterCacheTagsTest.php
@@ -108,6 +108,7 @@ public function testCacheTags() {
       'comment_view',
       'comment:' . $comment->id(),
       'config:filter.format.plain_text',
+      'config:user.settings',
       'user_view',
       'user:2',
     );