diff -u b/core/modules/file/lib/Drupal/file/Controller/FileWidgetAjaxController.php b/core/modules/file/lib/Drupal/file/Controller/FileWidgetAjaxController.php
--- b/core/modules/file/lib/Drupal/file/Controller/FileWidgetAjaxController.php
+++ b/core/modules/file/lib/Drupal/file/Controller/FileWidgetAjaxController.php
@@ -21,10 +21,10 @@
 class FileWidgetAjaxController extends FormAjaxController {
 
   /**
-   * Processes AJAX file uploads.
+   * Processes AJAX file uploads and deletions.
    *
    * @param \Symfony\Component\HttpFoundation\Request $request
-   *  The current request object.
+   *   The current request object.
    *
    * @return \Drupal\Core\Ajax\AjaxResponse
    *   An AjaxResponse object.
diff -u b/core/modules/system/lib/Drupal/system/Controller/FormAjaxController.php b/core/modules/system/lib/Drupal/system/Controller/FormAjaxController.php
--- b/core/modules/system/lib/Drupal/system/Controller/FormAjaxController.php
+++ b/core/modules/system/lib/Drupal/system/Controller/FormAjaxController.php
@@ -12,18 +12,23 @@
 use Symfony\Component\HttpKernel\Exception\HttpException;
 
 /**
- * Defines a controller to respond to form AJAX requests.
+ * Defines a controller to respond to form Ajax requests.
  */
 class FormAjaxController {
 
   /**
-   * Handle form AJAX request.
+   * Processes an Ajax form submission.
    *
    * @param \Symfony\Component\HttpFoundation\Request $request
-   *  The current request object.
+   *   The current request object.
    *
-   * @return \Symfony\Component\HttpFoundation\Response
-   *   A Symfony response object.
+   * @return mixed
+   *   Whatever is returned by the triggering element's #ajax['callback']
+   *   function. One of:
+   *   - A render array containing the new or updated content to return to the
+   *     browser. This is commonly an element within the rebuilt form.
+   *   - A \Drupal\Core\Ajax\AjaxResponse object containing commands for the
+   *     browser to process.
    *
    * @throws \Symfony\Component\HttpKernel\Exception\HttpExceptionInterface
    */
@@ -53,7 +58,7 @@
    * pulls the form info from the request body.
    *
    * @param \Symfony\Component\HttpFoundation\Request $request
-   *  The current request object.
+   *   The current request object.
    *
    * @return array
    *   An array containing the $form and $form_state. Use the list() function
diff -u b/core/modules/system/lib/Drupal/system/Tests/Ajax/FormValuesTest.php b/core/modules/system/lib/Drupal/system/Tests/Ajax/FormValuesTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/Ajax/FormValuesTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Ajax/FormValuesTest.php
@@ -53,17 +53,13 @@
     }
 
-    // Verify that an AJAX element without a callback throws error code 500.
-    $edit = array(
-      'select_empty_callback' => 'red',
-    );
-    $commands = $this->drupalPostAJAX('ajax_forms_test_get_form', $edit, 'select_empty_callback');
-    $this->assertResponse(500);
-
-    // Verify that an AJAX element with a nonexistent callback throws error code 500.
-    $edit = array(
-      'select_nonexistent_callback' => 'red',
-    );
-    $commands = $this->drupalPostAJAX('ajax_forms_test_get_form', $edit, 'select_nonexistent_callback');
-    $this->assertResponse(500);
+    // Verify that AJAX elements with invalid callbacks return error code 500.
+    foreach (array('null', 'empty', 'nonexistent') as $key) {
+      $element_name = 'select_' . $key . '_callback';
+      $edit = array(
+        $element_name => 'red',
+      );
+      $commands = $this->drupalPostAJAX('ajax_forms_test_get_form', $edit, $element_name);
+      $this->assertResponse(500);
+    }
   }
 }
diff -u b/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
--- b/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
@@ -73,17 +73,20 @@
     '#value' => t('submit'),
   );
 
-  $form['select_empty_callback'] = array(
-    '#type' => 'select',
-    '#options' => array('red' => 'red'),
-    '#ajax' => array('callback' => ''),
-  );
-
-  $form['select_nonexistent_callback'] = array(
-    '#type' => 'select',
-    '#options' => array('red' => 'red'),
-    '#ajax' => array('callback' => 'some_function_that_does_not_exist'),
+  // This is for testing invalid callbacks that should return a 500 error in
+  // \Drupal\system\FormAjaxController::content().
+  $invalid_callbacks = array(
+    'null' => NULL,
+    'empty' => '',
+    'nonexistent' => 'some_function_that_does_not_exist',
   );
+  foreach ($invalid_callbacks as $key => $value) {
+    $form['select_' . $key . '_callback'] = array(
+      '#type' => 'select',
+      '#options' => array('red' => 'red'),
+      '#ajax' => array('callback' => $value),
+    );
+  }
 
   return $form;
 }