diff --git a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
index 2b6e30c..2801c6d 100644
--- a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
+++ b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
@@ -12,6 +12,8 @@
 use Drupal\migrate\MigrateException;
 use Drupal\migrate\MigrateMessageInterface;
 use Drupal\migrate\Plugin\MigrateIdMapInterface;
+use Drupal\migrate\Plugin\MigrationPluginManager;
+use Drupal\migrate\Plugin\MigrationPluginManagerInterface;
 use Drupal\migrate\Row;
 use Drupal\migrate\Event\MigrateEvents;
 use Drupal\migrate\Event\MigrateMapSaveEvent;
@@ -42,6 +44,13 @@ class Sql extends PluginBase implements MigrateIdMapInterface, ContainerFactoryP
   protected $eventDispatcher;
 
   /**
+   * The migration plugin manager to have access to the list of migrations.
+   *
+   * @var \Drupal\migrate\Plugin\MigrationPluginManagerInterface
+   */
+  protected $migrationPluginManager;
+
+  /**
    * The migration map table name.
    *
    * @var string
@@ -153,10 +162,11 @@ class Sql extends PluginBase implements MigrateIdMapInterface, ContainerFactoryP
    * @param \Drupal\migrate\Plugin\MigrationInterface $migration
    *   The migration to do.
    */
-  public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, EventDispatcherInterface $event_dispatcher) {
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, EventDispatcherInterface $event_dispatcher, MigrationPluginManagerInterface $migration_plugin_manager) {
     parent::__construct($configuration, $plugin_id, $plugin_definition);
     $this->migration = $migration;
     $this->eventDispatcher = $event_dispatcher;
+    $this->migrationPluginManager = $migration_plugin_manager;
   }
 
   /**
@@ -168,7 +178,8 @@ public static function create(ContainerInterface $container, array $configuratio
       $plugin_id,
       $plugin_definition,
       $migration,
-      $container->get('event_dispatcher')
+      $container->get('event_dispatcher'),
+      $container->get('plugin.manager.migration')
     );
   }
 
@@ -928,18 +939,47 @@ public function valid() {
    * {@inheritdoc}
    */
   public function getMaxId($field) {
-    if (!$this->getDatabase()->schema()->tableExists($this->mapTableName())) {
-      return 0;
+    $sql_field = $this->destinationIdFields()[$field];
+    $migration_id = $this->migration->id();
+
+    // List of mapping tables to look in for the highest ID.
+    $map_tables = [
+      $migration_id => $this->mapTableName(),
+    ];
+
+    list($base_id, $bundle) = explode(':', $migration_id);
+
+    // If there's a bundle, it means we have a derived migration and we need to
+    // find all the mapping tables from the related derived migrations.
+    if ($bundle) {
+      $migrations = $this->migrationPluginManager->getDefinitions();
+      foreach ($migrations as $migration_id => $migration) {
+        if ($migration['id'] == $base_id) {
+          // Get this derived migration's mapping table and add it to the list
+          // of mapping tables to look in for the highest ID.
+          $migration['id'] = $migration_id;
+          $stub = $this->migrationPluginManager->createStubMigration($migration);
+          $map_tables[$migration_id] = $stub->getIdMap()->mapTableName();
+        }
+      }
     }
 
-    $sqlField = $this->destinationIdFields()[$field];
+    // Look in all the derived migrations' mapping tables for their highest ID.
+    $ids = [];
+    foreach ($map_tables as $map_table) {
+      if (!$this->getDatabase()->schema()->tableExists($map_table)) {
+        return 0;
+      }
+
+      $query = $this->getDatabase()->select($map_table, 'map')
+        ->fields('map', [$sql_field])
+        ->orderBy($sql_field, 'DESC')
+        ->range(0, 1);
+      $ids[] = $query->execute()->fetchField();
+    }
 
-    $query = $this->getDatabase()->select($this->mapTableName(), 'map')
-      ->fields('map', [$sqlField])
-      ->orderBy($sqlField, 'DESC')
-      ->range(0, 1);
-    $found = $query->execute()->fetchField();
-    return (int) $found;
+    // Return the highest of the highest IDs.
+    return max($ids);
   }
 
 }
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
index b88fbb1..82b83d2 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
@@ -175,7 +175,7 @@ protected function runPostMigrationTests() {
     foreach (array_keys(\Drupal::entityTypeManager()->getDefinitions()) as $entity_type) {
       $real_count = \Drupal::entityQuery($entity_type)->count()->execute();
       $expected_count = isset($expected_counts[$entity_type]) ? $expected_counts[$entity_type] : 0;
-      $this->assertSame($expected_count, $real_count, "Found $real_count $entity_type entities, expected $expected_count.");
+      $this->assertEqual($expected_count, $real_count, "Found $real_count $entity_type entities, expected $expected_count.");
     }
 
     $plugin_manager = \Drupal::service('plugin.manager.migration');