diff --git a/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentSourceWithHighWaterTest.php b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentSourceWithHighWaterTest.php
index 290550cdb4..1813855f3f 100644
--- a/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentSourceWithHighWaterTest.php
+++ b/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d6/CommentSourceWithHighWaterTest.php
@@ -2,7 +2,11 @@
 
 namespace Drupal\Tests\comment\Kernel\Plugin\migrate\source\d6;
 
+use Drupal\Core\Database\Connection;
+use Drupal\migrate\Plugin\migrate\id_map\Sql;
+use Drupal\migrate\Plugin\MigrateIdMapInterface;
 use Drupal\Tests\migrate\Kernel\MigrateSqlSourceTestBase;
+use Prophecy\Argument;
 
 /**
  * Tests the Drupal 6 comment source w/ high water handling.
@@ -18,6 +22,46 @@ class CommentSourceWithHighWaterTest extends MigrateSqlSourceTestBase {
    */
   protected static $modules = ['comment', 'migrate_drupal'];
 
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    // Ensure that \Drupal\migrate\Plugin\migrate\source\SqlBase::mapJoinable()
+    // considers the ID map plugin not joinable.
+    $connection = $this->prophesize(Connection::class);
+    $connection->getConnectionOptions()->willReturn([
+      'driver' => 'foo',
+      'database' => 'bar',
+    ]);
+
+    $id_map = $this->prophesize(Sql::class);
+    $id_map->getDatabase()->willReturn(
+      $connection->reveal()
+    );
+    // Pretend the row with source ID 1 has already been imported; this is
+    // reflected in the high water mark. We consider none of the other rows
+    // imported, so those will return the empty array.
+    $id_map->getRowBySource(Argument::exact(['cid' => '1']))->willReturn([
+      'source_row_status' => MigrateIdMapInterface::STATUS_IMPORTED,
+    ]);
+    $id_map->getRowBySource(Argument::any())->willReturn([]);
+    // ::saveIdMapping() gets called when MigrateSourceTestBase::testSource()
+    // re-iterates over all rows while intentionally triggering a skip exception
+    // using migrate_skip_all_rows_test_migrate_prepare_row().
+    $id_map->saveIdMapping(
+      Argument::any(),
+      Argument::any(),
+      Argument::exact(MigrateIdMapInterface::STATUS_IGNORED)
+    )->willReturn();
+    // @todo This should not be necessary.
+    $id_map->delete(Argument::any(), Argument::any())->willReturn();
+    $this->migration->getIdMap()->willReturn(
+      $id_map->reveal()
+    );
+  }
+
   /**
    * {@inheritdoc}
    */