diff --git a/core/modules/config/config.test b/core/modules/config/config.test
index 26092d1..f0e7df0 100644
--- a/core/modules/config/config.test
+++ b/core/modules/config/config.test
@@ -346,13 +346,15 @@ class ConfigUpgradeTestCase extends WebTestBase {
       ->values(array('config_upgrade_bar', serialize($this->testContent)))
       ->execute();
 
+    // Perform migration.
     update_variables_to_config('config_upgrade.test', array(
       'config_upgrade_bar' => 'parent.bar',
       'config_upgrade_foo' => 'foo',
-      // Non-existing variable.
+      // A default configuration value for which no variable exists.
       'config_upgrade_baz' => 'parent.baz',
     ));
 
+    // Verify that variables have been converted and default values exist.
     $config = config('config_upgrade.test');
     $this->assertIdentical($config->get('foo'), $this->testContent);
     $this->assertIdentical($config->get('parent.bar'), $this->testContent);
@@ -361,5 +363,27 @@ class ConfigUpgradeTestCase extends WebTestBase {
     // Verify that variables have been deleted.
     $variables = db_query('SELECT name FROM {variable} WHERE name IN (:names)', array(':names' => array('config_upgrade_bar', 'config_upgrade_foo')))->fetchCol();
     $this->assertFalse($variables);
+
+    // Add another variable to migrate into the same config object.
+    db_insert('variable')
+      ->fields(array('name', 'value'))
+      ->values(array('config_upgrade_additional', serialize($this->testContent)))
+      ->execute();
+
+    // Perform migration into the exsting config object.
+    update_variables_to_config('config_upgrade.test', array(
+      'config_upgrade_additional' => 'parent.additional',
+    ));
+
+    // Verify that new variables have been converted and existing still exist.
+    $config = config('config_upgrade.test');
+    $this->assertIdentical($config->get('foo'), $this->testContent);
+    $this->assertIdentical($config->get('parent.bar'), $this->testContent);
+    $this->assertIdentical($config->get('parent.baz'), 'Baz');
+    $this->assertIdentical($config->get('parent.additional'), $this->testContent);
+
+    // Verify that variables have been deleted.
+    $variables = db_query('SELECT name FROM {variable} WHERE name IN (:names)', array(':names' => array('config_upgrade_additional')))->fetchCol();
+    $this->assertFalse($variables);
   }
 }