diff --git a/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php b/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php
index 13f0042..9361b87 100644
--- a/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php
+++ b/core/lib/Drupal/Core/Form/FormStateDecoratorBase.php
@@ -469,55 +469,6 @@ public function &getValues() {
   /**
    * {@inheritdoc}
    */
-  public function &getValue($key, $default = NULL) {
-    return $this->decoratedFormState->getValue($key, $default);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setValues(array $values) {
-    return $this->decoratedFormState->setValues($values);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setValue($key, $value) {
-    return $this->decoratedFormState->setValue($key, $value);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function unsetValue($key) {
-    return $this->decoratedFormState->unsetValue($key);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function hasValue($key) {
-    return $this->decoratedFormState->hasValue($key);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isValueEmpty($key) {
-    return $this->decoratedFormState->isValueEmpty($key);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setValueForElement(array $element, $value) {
-    return $this->decoratedFormState->getValues($element, $value);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function setResponse(Response $response) {
     return $this->decoratedFormState->setResponse($response);
   }
@@ -554,7 +505,7 @@ public function getRedirect() {
    * {@inheritdoc}
    */
   public static function hasAnyErrors() {
-    return FormStateManager::hasAnyErrors();
+    return FormState::hasAnyErrors();
   }
 
   /**
@@ -638,7 +589,7 @@ public function getCleanValueKeys() {
    * {@inheritdoc}
    */
   public function setCleanValueKeys(array $cleanValueKeys) {
-    return $this->decoratedFormState->cleanValues($cleanValueKeys);
+    return $this->decoratedFormState->setCleanValueKeys($cleanValueKeys);
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Form/SubFormState.php b/core/lib/Drupal/Core/Form/SubFormState.php
index a27ad87..dbdb7b9 100644
--- a/core/lib/Drupal/Core/Form/SubFormState.php
+++ b/core/lib/Drupal/Core/Form/SubFormState.php
@@ -8,11 +8,12 @@
 namespace Drupal\Core\Form;
 
 use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Render\Element;
 
 /**
  * Stores information about the state of a subform.
  */
-class SubFormState extends FormStateDecoratorBase implements FormStateInterface {
+class SubFormState extends FormStateDecoratorBase {
 
   use FormStateValuesTrait;
 
@@ -55,9 +56,17 @@ public function __construct(FormStateInterface $parent_form_state, array $array_
    *   The element to create the sub form state for.
    *
    * @return static
+   *
+   * @throws \InvalidArgumentException
+   *   Thrown when the form element does not wrap other form elements, or if it
+   *   does not have the necessary Form API properties assigned.
    */
   public static function createForFormElement(FormStateInterface $parent_form_state, array $element) {
     // Make sure the element contains the required properties.
+    $children = Element::children($element);
+    if (empty($children)) {
+      throw new \InvalidArgumentException('nope');
+    }
     $required_keys = ['#array_parents', '#parents'];
     foreach ($required_keys as $required_key) {
       if (!array_key_exists($required_key, $element) || !is_array($element[$required_key])) {
diff --git a/core/tests/Drupal/Tests/Core/Form/FormStateTest.php b/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
index 3b5ab7a..4fca585 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormStateTest.php
@@ -427,6 +427,7 @@ public function testGetFormStateForElement() {
     $element = [
       '#parents' => ['foo'],
       '#array_parents' => ['foo'],
+      'bar' => [],
     ];
 
     $sub_form_state = $form_state->getFormStateForElement($element);
diff --git a/core/tests/Drupal/Tests/Core/Form/SubFormStateTest.php b/core/tests/Drupal/Tests/Core/Form/SubFormStateTest.php
index e782cdd..55c060a 100644
--- a/core/tests/Drupal/Tests/Core/Form/SubFormStateTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/SubFormStateTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Tests\Core\Form;
 
+use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Form\FormState;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Tests\UnitTestCase;
@@ -30,6 +31,24 @@ class SubFormStateTest extends UnitTestCase {
       'name' => 'Dodger',
     ],
   ];
+  protected $element = [
+    'foo' => [
+      '#parents' => ['foo'],
+      '#array_parents' => ['foo'],
+    ],
+    'dog' => [
+      '#parents' => ['dog'],
+      '#array_parents' => ['dog'],
+      'breed' => [
+        '#parents' => ['dog', 'breed'],
+        '#array_parents' => ['dog', 'breed'],
+      ],
+      'name' => [
+        '#parents' => ['dog', 'name'],
+        '#array_parents' => ['dog', 'name'],
+      ],
+    ],
+  ];
 
   /**
    * @covers ::createForFormElement
@@ -39,6 +58,7 @@ public function testCreateForFormElement() {
     $element = [
       '#parents' => ['foo'],
       '#array_parents' => ['foo'],
+      'bar' => [],
     ];
 
     $sub_form_state = $form_state->getFormStateForElement($element);
@@ -54,7 +74,9 @@ public function testCreateForFormElement() {
    */
   public function testCreateForFormElementMissingArrayParents() {
     $form_state = new FormState();
-    $element = [];
+    $element = [
+      'bar' => [],
+    ];
 
     $form_state->getFormStateForElement($element);
   }
@@ -69,6 +91,7 @@ public function testCreateForFormElementMissingParents() {
     $form_state = new FormState();
     $element = [
       '#array_parents' => ['foo'],
+      'bar' => [],
     ];
 
     $form_state->getFormStateForElement($element);
@@ -82,11 +105,8 @@ public function testCreateForFormElementMissingParents() {
   public function testGetValues($parents, $expected) {
     $form_state = new FormState();
     $form_state->setValues($this->initialValues);
-    $element = [
-      '#parents' => $parents,
-      '#array_parents' => $parents,
-    ];
 
+    $element = NestedArray::getValue($this->element, $parents);
     $sub_form_state = $form_state->getFormStateForElement($element);
     $sub_values = $sub_form_state->getValues();
     $this->assertSame($expected, $sub_values);
@@ -94,26 +114,34 @@ public function testGetValues($parents, $expected) {
 
   public function providerTestGetValues() {
     $data = [];
-    $data['exist1'] = [
+    $data['exist'] = [
       ['dog'],
       $this->initialValues['dog'],
     ];
-    $data['exist2'] = [
+    return $data;
+  }
+
+  /**
+   * @covers ::getValues
+   *
+   * @dataProvider providerTestGetValuesBroken
+   *
+   * @expectedException \InvalidArgumentException
+   */
+  public function testGetValuesBroken($parents, $expected) {
+    $this->testGetValues($parents, $expected);
+  }
+
+  public function providerTestGetValuesBroken() {
+    $data = [];
+    $data['exist'] = [
       ['foo'],
       $this->initialValues['foo'],
     ];
-    $data['not_exist'] = [
-      ['cat'],
-      [],
-    ];
     $data['nested'] = [
       ['dog', 'name'],
       'Dodger',
     ];
-    $data['empty_parents'] = [
-      [],
-      $this->initialValues,
-    ];
     return $data;
   }
 
@@ -125,11 +153,8 @@ public function providerTestGetValues() {
   public function testGetValue($parents, $key, $expected, $default = NULL) {
     $form_state = new FormState();
     $form_state->setValues($this->initialValues);
-    $element = [
-      '#parents' => $parents,
-      '#array_parents' => $parents,
-    ];
 
+    $element = NestedArray::getValue($this->element, $parents);
     $sub_form_state = $form_state->getFormStateForElement($element);
     $sub_values = $sub_form_state->getValue($key, $default);
     $this->assertSame($expected, $sub_values);
@@ -142,32 +167,27 @@ public function providerTestGetValue() {
       'name',
       'Dodger',
     ];
-    $data['not_exist'] = [
-      ['cat'],
-      'cat',
-      NULL,
-    ];
+    return $data;
+  }
+
+  /**
+   * @covers ::getValue
+   *
+   * @dataProvider providerTestGetValueBroken
+   *
+   * @expectedException \InvalidArgumentException
+   */
+  public function testGetValueBroken($parents, $key, $expected, $default = NULL) {
+    $this->testGetValue($parents, $key, $expected, $default);
+  }
+
+  public function providerTestGetValueBroken() {
+    $data = [];
     $data['nested'] = [
       ['dog', 'name'],
       NULL,
       'Dodger',
     ];
-    $data['not_exist_with_default'] = [
-      ['cat'],
-      'cat',
-      'Steak',
-      'Steak',
-    ];
-    $data['empty_parents_null_key'] = [
-      [],
-      NULL,
-      $this->initialValues,
-    ];
-    $data['empty_parents'] = [
-      [],
-      'dog',
-      $this->initialValues['dog'],
-    ];
     return $data;
   }
 
@@ -179,11 +199,8 @@ public function providerTestGetValue() {
   public function testSetValues($parents, $new_values, $expected) {
     $form_state = new FormState();
     $form_state->setValues($this->initialValues);
-    $element = [
-      '#parents' => $parents,
-      '#array_parents' => $parents,
-    ];
 
+    $element = NestedArray::getValue($this->element, $parents);
     $sub_form_state = $form_state->getFormStateForElement($element);
     $sub_form_state->setValues($new_values);
     $this->assertSame($expected, $form_state->getValues());
@@ -191,7 +208,7 @@ public function testSetValues($parents, $new_values, $expected) {
 
   public function providerTestSetValues() {
     $data = [];
-    $data['exist1'] = [
+    $data['exist'] = [
       ['dog'],
       [],
       [
@@ -199,7 +216,23 @@ public function providerTestSetValues() {
         'dog' => [],
       ],
     ];
-    $data['exist2'] = [
+    return $data;
+  }
+
+  /**
+   * @covers ::setValues
+   *
+   * @dataProvider providerTestSetValuesBroken
+   *
+   * @expectedException \InvalidArgumentException
+   */
+  public function testSetValuesBroken($parents, $new_values, $expected) {
+    $this->testSetValues($parents, $new_values, $expected);
+  }
+
+  public function providerTestSetValuesBroken() {
+    $data = [];
+    $data['exist'] = [
       ['foo'],
       [],
       [
@@ -207,11 +240,6 @@ public function providerTestSetValues() {
         'dog' => $this->initialValues['dog'],
       ],
     ];
-    $data['not_exist'] = [
-      ['cat'],
-      [],
-      $this->initialValues,
-    ];
     return $data;
   }