diff --git a/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php b/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
index 174d1bb..35d6e85 100644
--- a/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
+++ b/core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php
@@ -11,6 +11,7 @@
 use Drupal\Core\Form\FormAjaxResponseBuilderInterface;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
 use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\Exception\HttpException;
 use Symfony\Component\HttpKernel\KernelEvents;
 
 /**
@@ -19,11 +20,6 @@
 class FormAjaxSubscriber implements EventSubscriberInterface {
 
   /**
-   * Request key for a form that will post to original URL on AJAX submission.
-   */
-  const POST_TO_ORIGINAL_URL = 'post_to_original_url';
-
-  /**
    * The form AJAX response builder.
    *
    * @var \Drupal\Core\Form\FormAjaxResponseBuilderInterface
@@ -56,11 +52,18 @@ public function onException(GetResponseForExceptionEvent $event) {
       // Set the build ID from the request as the old build ID on the form.
       $form['#build_id_old'] = $request->get('form_build_id');
 
-      $response = $this->formAjaxResponseBuilder->buildResponse($request, $form, $form_state, []);
-      // Since this response is being set in place of an exception, explicitly
-      // mark this as a 200 status.
-      $response->headers->set('X-Status-Code', 200);
-      $event->setResponse($response);
+      try {
+        $response = $this->formAjaxResponseBuilder->buildResponse($request, $form, $form_state, []);
+
+        // Since this response is being set in place of an exception, explicitly
+        // mark this as a 200 status.
+        $response->headers->set('X-Status-Code', 200);
+        $event->setResponse($response);
+      }
+      catch (HttpException $e) {
+        // Otherwise, replace the existing exception with the new one.
+        $event->setException($e);
+      }
     }
   }
 
diff --git a/core/lib/Drupal/Core/Form/FormAjaxResponseBuilderInterface.php b/core/lib/Drupal/Core/Form/FormAjaxResponseBuilderInterface.php
index 8822a33..720ecc8 100644
--- a/core/lib/Drupal/Core/Form/FormAjaxResponseBuilderInterface.php
+++ b/core/lib/Drupal/Core/Form/FormAjaxResponseBuilderInterface.php
@@ -28,6 +28,9 @@
    *
    * @return \Drupal\Core\Ajax\AjaxResponse
    *   An AJAX response representing the form and its AJAX commands.
+   *
+   * @throws \Symfony\Component\HttpKernel\Exception\HttpException
+   *   Thrown if the AJAX callback is not a callable.
    */
   public function buildResponse(Request $request, array $form, FormStateInterface $form_state, array $commands);
 
diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index ea1e3ab..382bba1 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -14,7 +14,6 @@
 use Drupal\Component\Utility\UrlHelper;
 use Drupal\Core\Access\CsrfTokenGenerator;
 use Drupal\Core\DependencyInjection\ClassResolverInterface;
-use Drupal\Core\Form\EventSubscriber\FormAjaxSubscriber;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Render\ElementInfoManagerInterface;
@@ -247,7 +246,7 @@ public function buildForm($form_id, FormStateInterface &$form_state) {
 
     // If this form should post to the original URL, disable all form redirects.
     $request = $this->requestStack->getCurrentRequest();
-    if ($post_to_original_url = $request->query->has(FormAjaxSubscriber::POST_TO_ORIGINAL_URL)) {
+    if ($post_to_original_url = $request->query->has(static::POST_TO_ORIGINAL_URL)) {
       $form_state->disableRedirect();
     }
 
@@ -265,8 +264,12 @@ public function buildForm($form_id, FormStateInterface &$form_state) {
     $response = $this->processForm($form_id, $form, $form_state);
 
     // After processing the form, if this is to be posted to the original URL,
-    // interrupt form rendering and return.
-    // @see \Drupal\Core\Form\EventSubscriber\FormAjaxSubscriber::onException()
+    // interrupt form rendering and return by throwing an exception that
+    // contains the processed form and form state. This exception will be caught
+    // by \Drupal\Core\Form\EventSubscriber\FormAjaxSubscriber::onException()
+    // and then passed through
+    // \Drupal\Core\Form\FormAjaxResponseBuilderInterface::buildResponse() to
+    // build a proper AJAX response.
     if ($post_to_original_url) {
       throw new FormAjaxException($form, $form_state);
     }
diff --git a/core/lib/Drupal/Core/Form/FormBuilderInterface.php b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
index 53641c6..76d3342 100644
--- a/core/lib/Drupal/Core/Form/FormBuilderInterface.php
+++ b/core/lib/Drupal/Core/Form/FormBuilderInterface.php
@@ -13,6 +13,11 @@
 interface FormBuilderInterface {
 
   /**
+   * Request key for a form that will post to original URL on AJAX submission.
+   */
+  const POST_TO_ORIGINAL_URL = 'post_to_original_url';
+
+  /**
    * Determines the ID of a form.
    *
    * @param \Drupal\Core\Form\FormInterface|string $form_arg
diff --git a/core/lib/Drupal/Core/Render/Element/RenderElement.php b/core/lib/Drupal/Core/Render/Element/RenderElement.php
index f82420e..623f418 100644
--- a/core/lib/Drupal/Core/Render/Element/RenderElement.php
+++ b/core/lib/Drupal/Core/Render/Element/RenderElement.php
@@ -7,7 +7,7 @@
 
 namespace Drupal\Core\Render\Element;
 
-use Drupal\Core\Form\EventSubscriber\FormAjaxSubscriber;
+use Drupal\Core\Form\FormBuilderInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Plugin\PluginBase;
 use Drupal\Core\Render\Element;
@@ -241,7 +241,7 @@ public static function preRenderAjaxForm($element) {
       // re-rendering. For that, we have a special 'system/ajax' route.
       if (isset($settings['callback']) && !isset($settings['url'])) {
         $settings['url'] = Url::fromRoute('<current>');
-        $settings['options']['query'][FormAjaxSubscriber::POST_TO_ORIGINAL_URL] = TRUE;
+        $settings['options']['query'][FormBuilderInterface::POST_TO_ORIGINAL_URL] = TRUE;
       }
       $settings += array(
         'url' => NULL,
diff --git a/core/modules/system/src/Tests/Ajax/AjaxFormCacheTest.php b/core/modules/system/src/Tests/Ajax/AjaxFormCacheTest.php
new file mode 100644
index 0000000..7d73c82
--- /dev/null
+++ b/core/modules/system/src/Tests/Ajax/AjaxFormCacheTest.php
@@ -0,0 +1,49 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Tests\Ajax\AjaxFormCacheTest.
+ */
+
+namespace Drupal\system\Tests\Ajax;
+
+use Drupal\Core\Url;
+
+/**
+ * Tests the usage of form caching for AJAX forms.
+ *
+ * @group Ajax
+ */
+class AjaxFormCacheTest extends AjaxTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function testFormCacheUsage() {
+    /** @var \Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface $key_value_expirable */
+    $key_value_expirable = \Drupal::service('keyvalue.expirable')->get('form');
+    $this->drupalLogin($this->rootUser);
+
+    // Ensure that the cache is empty.
+    $this->assertEqual(0, count($key_value_expirable->getAll()));
+
+    // Visit an AJAX form that is not cached, 3 times.
+    $uncached_form_url = Url::fromRoute('ajax_forms_test.commands_form');
+    $this->drupalGet($uncached_form_url);
+    $this->drupalGet($uncached_form_url);
+    $this->drupalGet($uncached_form_url);
+
+    // The number of cache entries should not have changed.
+    $this->assertEqual(0, count($key_value_expirable->getAll()));
+
+    // Visit a form that is explicitly cached, 3 times.
+    $cached_form_url = Url::fromRoute('ajax_forms_test.get_form');
+    $this->drupalGet($cached_form_url);
+    $this->drupalGet($cached_form_url);
+    $this->drupalGet($cached_form_url);
+
+    // The number of cache entries should be exactly 3.
+    $this->assertEqual(3, count($key_value_expirable->getAll()));
+  }
+
+}
diff --git a/core/modules/system/src/Tests/Ajax/DialogTest.php b/core/modules/system/src/Tests/Ajax/DialogTest.php
index 6ac88a9..603cfa0 100644
--- a/core/modules/system/src/Tests/Ajax/DialogTest.php
+++ b/core/modules/system/src/Tests/Ajax/DialogTest.php
@@ -7,8 +7,8 @@
 
 namespace Drupal\system\Tests\Ajax;
 
-use Drupal\Core\Form\EventSubscriber\FormAjaxSubscriber;
 use Drupal\Core\EventSubscriber\MainContentViewSubscriber;
+use Drupal\Core\Form\FormBuilderInterface;
 use Drupal\Core\Url;
 
 /**
@@ -166,7 +166,7 @@ public function testDialog() {
       'edit-preview' => [
         'callback' => '::preview',
         'event' => 'click',
-        'url' => Url::fromRoute('ajax_test.dialog_form')->setOption('query', [FormAjaxSubscriber::POST_TO_ORIGINAL_URL => TRUE])->toString(),
+        'url' => Url::fromRoute('ajax_test.dialog_form')->setOption('query', [FormBuilderInterface::POST_TO_ORIGINAL_URL => TRUE])->toString(),
         'dialogType' => 'ajax',
         'submit' => [
           '_triggering_element_name' => 'op',
diff --git a/core/tests/Drupal/Tests/Core/Form/EventSubscriber/FormAjaxSubscriberTest.php b/core/tests/Drupal/Tests/Core/Form/EventSubscriber/FormAjaxSubscriberTest.php
index 9f93b41..19adec8 100644
--- a/core/tests/Drupal/Tests/Core/Form/EventSubscriber/FormAjaxSubscriberTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/EventSubscriber/FormAjaxSubscriberTest.php
@@ -14,6 +14,7 @@
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\Response;
 use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\Exception\HttpException;
 use Symfony\Component\HttpKernel\HttpKernelInterface;
 
 /**
@@ -95,7 +96,6 @@ public function testOnExceptionNewBuildId() {
       ->with($request, $expected_form, $form_state, $commands)
       ->willReturn($response);
 
-
     $event = new GetResponseForExceptionEvent($this->httpKernel, $request, HttpKernelInterface::MASTER_REQUEST, $exception);
     $this->subscriber->onException($event);
 
@@ -119,4 +119,30 @@ public function testOnExceptionOtherClass() {
     $this->assertNull($event->getResponse());
   }
 
+  /**
+   * @covers ::onException
+   */
+  public function testOnExceptionResponseBuilderException() {
+    $form = ['#type' => 'form', '#build_id' => 'the_build_id'];
+    $expected_form = $form + [
+      '#build_id_old' => 'the_build_id',
+    ];
+    $form_state = new FormState();
+    $exception = new FormAjaxException($form, $form_state);
+    $request = new Request([], ['form_build_id' => 'the_build_id']);
+    $commands = [];
+
+    $expected_exception = new HttpException(500, 'The specified #ajax callback is empty or not callable.');
+    $this->formAjaxResponseBuilder->expects($this->once())
+      ->method('buildResponse')
+      ->with($request, $expected_form, $form_state, $commands)
+      ->willThrowException($expected_exception);
+
+    $event = new GetResponseForExceptionEvent($this->httpKernel, $request, HttpKernelInterface::MASTER_REQUEST, $exception);
+    $this->subscriber->onException($event);
+
+    $this->assertNull($event->getResponse());
+    $this->assertSame($expected_exception, $event->getException());
+  }
+
 }