diff -u b/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php b/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php
--- b/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php
+++ b/core/modules/migrate/lib/Drupal/migrate/Entity/Migration.php
@@ -38,9 +38,6 @@
  *     "weight" = "weight",
  *     "uuid" = "uuid"
  *   },
- *   links = {
- *     "edit-form" = "admin/config/migration/{migration_entity}"
- *   }
  * )
  */
 class Migration extends ConfigEntityBase implements MigrationInterface {
diff -u b/core/modules/migrate/lib/Drupal/migrate/MigrateExecutable.php b/core/modules/migrate/lib/Drupal/migrate/MigrateExecutable.php
--- b/core/modules/migrate/lib/Drupal/migrate/MigrateExecutable.php
+++ b/core/modules/migrate/lib/Drupal/migrate/MigrateExecutable.php
@@ -224,11 +224,11 @@
     }
     while ($source->valid()) {
       $row = $source->current();
-      $this->sourceIdValues = $row->getSourceIdValues();
-
-      // Wipe old messages, and save any new messages.
-      $id_map->delete($row->getSourceIdValues(), TRUE);
-      $this->saveQueuedMessages();
+      if ($this->sourceIdValues = $row->getSourceIdValues()) {
+        // Wipe old messages, and save any new messages.
+        $id_map->delete($row->getSourceIdValues(), TRUE);
+        $this->saveQueuedMessages();
+      }
 
       $this->processRow($row);
 
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/migrate/config/migrate.migration.d6_system_rss.yml
@@ -0,0 +1,10 @@
+id: d6_system_rss
+source:
+    plugin: drupal6_variable
+    variables:
+        - feed_default_items
+process:
+    'items:limit': feed_default_items
+destination:
+    plugin: d8_config
+    config_name: system.rss
only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/migrate/lib/Drupal/migrate/Tests/Dump/Drupal6SystemRss.php
@@ -0,0 +1,55 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\migrate\Tests\Drupal6SystemRss.
+ */
+
+namespace Drupal\migrate\Tests\Dump;
+
+use Drupal\Core\Database\Connection;
+
+/**
+ * Database dump for testing system.settings.yml migration.
+ */
+class Drupal6SystemRss {
+
+  /**
+   * Sample database schema and values.
+   *
+   * @param \Drupal\Core\Database\Connection $database
+   *   The database connection.
+   */
+  public static function load(Connection $database) {
+    $database->schema()->createTable('variable', array(
+      'fields' => array(
+        'name' => array(
+          'type' => 'varchar',
+          'length' => 128,
+          'not null' => TRUE,
+          'default' => '',
+        ),
+        'value' => array(
+          'type' => 'blob',
+          'not null' => TRUE,
+          'size' => 'big',
+          'translatable' => TRUE,
+        ),
+      ),
+      'primary key' => array(
+        'name',
+      ),
+      'module' => 'system',
+      'name' => 'variable',
+    ));
+    $database->insert('variable')->fields(array(
+      'name',
+      'value',
+    ))
+    ->values(array(
+      'name' => 'feed_default_items',
+      'value' => 'i:10;',
+    ))
+    ->execute();
+  }
+}