diff --git a/core/modules/comment/src/Plugin/migrate/source/d7/Comment.php b/core/modules/comment/src/Plugin/migrate/source/d7/Comment.php
index 73c2d8f4f7..d5b9a11ac2 100644
--- a/core/modules/comment/src/Plugin/migrate/source/d7/Comment.php
+++ b/core/modules/comment/src/Plugin/migrate/source/d7/Comment.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\comment\Plugin\migrate\source\d7;
 
+use Drupal\Core\Language\LanguageInterface;
 use Drupal\migrate\Row;
 use Drupal\migrate_drupal\Plugin\migrate\source\d7\FieldableEntity;
 
@@ -44,6 +45,12 @@ public function prepareRow(Row $row) {
     $source_language = $this->getEntityTranslationSourceLanguage('comment', $cid);
     $language = $entity_translatable && $source_language ? $source_language : $row->getSourceProperty('language');
 
+    // If this comment was upgraded from Drupal 6 or older, it's possible that
+    // no language code was specified.
+    if (!$row->getSourceProperty('language')) {
+      $row->setSourceProperty('language', LanguageInterface::LANGCODE_NOT_SPECIFIED);
+    }
+
     // Get Field API field values.
     foreach ($this->getFields('comment', $comment_type) as $field_name => $field) {
       // Ensure we're using the right language if the entity and the field are