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 6bdd51e..f601e0f 100644
--- a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
+++ b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
@@ -576,7 +576,7 @@ public function lookupDestinationIds(array $source_id_values) {
     }
 
     if (!empty($source_id_values)) {
-      throw new MigrateException("Extra unknown items in source IDs");
+      throw new MigrateException(sprintf("Extra unknown items in source IDs: %s", var_dump($source_id_values)));
     }
 
     $query = $this->getDatabase()->select($this->mapTableName(), 'map')