diff --git a/core/includes/ajax.inc b/core/includes/ajax.inc
index 480f9b8..bd55bc4 100644
--- a/core/includes/ajax.inc
+++ b/core/includes/ajax.inc
@@ -1209,4 +1209,3 @@ function ajax_command_add_css($styles) {
     'data' => $styles,
   );
 }
-
diff --git a/core/modules/system/lib/Drupal/system/Tests/Ajax/CommandsTest.php b/core/modules/system/lib/Drupal/system/Tests/Ajax/CommandsTest.php
index 5b137d3..faa35bb 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Ajax/CommandsTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Ajax/CommandsTest.php
@@ -162,7 +162,7 @@ class CommandsTest extends AjaxTestBase {
     $commands = $this->drupalPostAJAX($form_path, $edit, array('op' => t("AJAX 'add_css' command")));
     $expected = array(
       'command' => 'add_css',
-      'data' => array('my/file.css'),
+      'data' => 'my/file.css',
     );
     $this->assertCommand($commands, $expected, "'add_css' AJAX command issued with correct data");
   }
diff --git a/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module b/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module
index 6a95710..d4074d1 100644
--- a/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module
+++ b/core/modules/system/tests/modules/ajax_forms_test/ajax_forms_test.module
@@ -258,6 +258,15 @@ function ajax_forms_test_ajax_commands_form($form, &$form_state) {
     ),
   );
 
+  // Shows the Ajax 'add_css' command.
+  $form['add_css_command_example'] = array(
+    '#type' => 'submit',
+    '#value' => t("AJAX 'add_css' command"),
+    '#ajax' => array(
+      'callback' => 'ajax_forms_test_advanced_commands_add_css_callback',
+    ),
+  );
+
   $form['submit'] = array(
     '#type' => 'submit',
     '#value' => t('Submit'),
@@ -411,6 +420,15 @@ function ajax_forms_test_advanced_commands_settings_callback($form, $form_state)
 }
 
 /**
+ * Ajax callback for 'add_css'.
+ */
+function ajax_forms_test_advanced_commands_add_css_callback($form, $form_state) {
+  $commands = array();
+  $commands[] = ajax_command_add_css('my/file.css');
+  return array('#type' => 'ajax', '#commands' => $commands);
+}
+
+/**
  * This form and its related submit and callback functions demonstrate
  * not validating another form element when a single Ajax element is triggered.
  *