diff --git a/core/modules/config/lib/Drupal/config/Tests/ConfigUpgradeTest.php b/core/modules/config/lib/Drupal/config/Tests/ConfigUpgradeTest.php
index 6fe07de..f55921e 100644
--- a/core/modules/config/lib/Drupal/config/Tests/ConfigUpgradeTest.php
+++ b/core/modules/config/lib/Drupal/config/Tests/ConfigUpgradeTest.php
@@ -98,5 +98,26 @@ function testConfigurationUpdate() {
     catch (ConfigException $e) {
       $this->pass('Exception was thrown on missing default module configuration file.');
     }
+
+    // For this test it is essential that update_variables_to_config has already
+    // run on the config object.
+    config('config_upgrade.test')
+      ->set('numeric_keys.403', '')
+      ->set('numeric_keys.404', '')
+      ->save();
+
+    db_insert('variable')
+      ->fields(array('name', 'value'))
+      ->values(array('config_upgrade_403', serialize('custom403')))
+      ->values(array('config_upgrade_404', serialize('custom404')))
+      ->execute();
+
+    // Perform migration.
+    update_variables_to_config('config_upgrade.test', array(
+      'config_upgrade_403' => 'numeric_keys.403',
+      'config_upgrade_404' => 'numeric_keys.404',
+    ));
+
+    $this->assertIdentical(config('config_upgrade.test')->get('numeric_keys'), array(403 => 'custom403', 404 => 'custom404'));
   }
 }
diff --git a/core/modules/system/tests/modules/config_upgrade/config/config_upgrade.test.yml b/core/modules/system/tests/modules/config_upgrade/config/config_upgrade.test.yml
index c921534..fc448c1 100644
--- a/core/modules/system/tests/modules/config_upgrade/config/config_upgrade.test.yml
+++ b/core/modules/system/tests/modules/config_upgrade/config/config_upgrade.test.yml
@@ -2,3 +2,6 @@ parent:
   bar: Bar
   baz: Baz
 foo: Foo
+numeric_keys:
+  403: ''
+  404: ''