diff --git a/core/modules/comment/templates/comment.html.twig b/core/modules/comment/templates/comment.html.twig
index 24dbd07..54087b9 100644
--- a/core/modules/comment/templates/comment.html.twig
+++ b/core/modules/comment/templates/comment.html.twig
@@ -71,7 +71,7 @@
     'comment',
     status != 'published' ? status,
     not author_id ? 'by-anonymous',
-    author_id and author_id == commented_entity.getOwnerId() ? 'by-' ~ commented_entity.getEntityTypeId() ~ '-author',
+    author_id and commented_entity.getOwnerId() is defined and author_id == commented_entity.getOwnerId() ? 'by-' ~ commented_entity.getEntityTypeId() ~ '-author',
     'clearfix',
   ]
 %}
diff --git a/core/themes/bartik/templates/comment.html.twig b/core/themes/bartik/templates/comment.html.twig
index 0fa1991..719e731 100644
--- a/core/themes/bartik/templates/comment.html.twig
+++ b/core/themes/bartik/templates/comment.html.twig
@@ -69,7 +69,7 @@
     'comment',
     status != 'published' ? status,
     not author_id ? 'by-anonymous',
-    author_id and author_id == commented_entity.getOwnerId() ? 'by-' ~ commented_entity.getEntityTypeId() ~ '-author',
+    author_id and commented_entity.getOwnerId() is defined and author_id == commented_entity.getOwnerId() ? 'by-' ~ commented_entity.getEntityTypeId() ~ '-author',
     'clearfix',
   ]
 %}