diff -u b/core/modules/migrate_drupal/src/Plugin/migrate/source/d6/Comment.php b/core/modules/migrate_drupal/src/Plugin/migrate/source/d6/Comment.php
--- b/core/modules/migrate_drupal/src/Plugin/migrate/source/d6/Comment.php
+++ b/core/modules/migrate_drupal/src/Plugin/migrate/source/d6/Comment.php
@@ -37,7 +37,7 @@
   /**
    * {@inheritdoc}
    */
-  public function prepareRow(Row $row, $keep = TRUE) {
+  public function prepareRow(Row $row) {
     if ($this->variableGet('comment_subject_field_' . $row->getSourceProperty('type'), 1)) {
       // Comment subject visible.
       $row->setSourceProperty('field_name', 'comment');