diff --git a/core/lib/Drupal/Core/Form/SubformState.php b/core/lib/Drupal/Core/Form/SubformState.php
index 50f3911..14cfd10 100644
--- a/core/lib/Drupal/Core/Form/SubformState.php
+++ b/core/lib/Drupal/Core/Form/SubformState.php
@@ -98,7 +98,12 @@ protected function getParents($property) {
    */
   public function &getValues() {
     $exists = NULL;
-    $values = &NestedArray::getValue(parent::getValues(), $this->getParents('#parents'), $exists);
+    if ($this->isProcessingInput()) {
+      $values = &NestedArray::getValue(parent::getValues(), $this->getParents('#parents'), $exists);
+    }
+    else {
+      @trigger_error("Please don't.", E_USER_DEPRECATED);
+    }
     if (!$exists) {
       $values = [];
     }
diff --git a/core/tests/Drupal/Tests/Core/Form/SubformStateTest.php b/core/tests/Drupal/Tests/Core/Form/SubformStateTest.php
index ec2ac8b..f485a6b 100644
--- a/core/tests/Drupal/Tests/Core/Form/SubformStateTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/SubformStateTest.php
@@ -62,10 +62,12 @@ class SubformStateTest extends UnitTestCase {
    *
    * @param string[] $parents
    * @param string $expected
+   * @param bool $is_processing_input
    */
-  public function testGetValues(array $parents, $expected) {
+  public function testGetValues(array $parents, $expected, $is_processing_input = TRUE) {
     $parent_form_state = new FormState();
     $parent_form_state->setValues($this->formStateValues);
+    $parent_form_state->setProcessInput($is_processing_input);
 
     $subform = NestedArray::getValue($this->parentForm, $parents);
     $subform_state = SubformState::createForSubform($subform, $this->parentForm, $parent_form_state);
@@ -73,9 +75,10 @@ public function testGetValues(array $parents, $expected) {
     $this->assertSame($expected, $subform_state_values);
 
     // Modify the retrieved values and confirm they are modified by reference in
-    // the parent form state.
+    // the parent form state when input is being processed.
     $subform_state_values['fish'] = 'Jim';
-    $this->assertSame($subform_state_values, $subform_state->getValues());
+    $subform_expected = $is_processing_input ? $subform_state_values : [];
+    $this->assertSame($subform_expected, $subform_state->getValues());
   }
 
   /**
@@ -87,6 +90,11 @@ public function providerGetValues() {
       ['dog'],
       $this->formStateValues['dog'],
     ];
+    $data['not_processing_input'] = [
+      ['dog'],
+      [],
+      FALSE,
+    ];
 
     return $data;
   }
@@ -131,6 +139,7 @@ public function providerGetValuesBroken() {
   public function testGetValue($parents, $key, $expected, $default = NULL) {
     $parent_form_state = new FormState();
     $parent_form_state->setValues($this->formStateValues);
+    $parent_form_state->setProcessInput();
 
     $subform = NestedArray::getValue($this->parentForm, $parents);
     $subform_state = SubformState::createForSubform($subform, $this->parentForm, $parent_form_state);
@@ -190,6 +199,7 @@ public function providerTestGetValueBroken() {
   public function testSetValues($parents, $new_values, $expected) {
     $parent_form_state = new FormState();
     $parent_form_state->setValues($this->formStateValues);
+    $parent_form_state->setProcessInput();
 
     $subform = NestedArray::getValue($this->parentForm, $parents);
     $subform_state = SubformState::createForSubform($subform, $this->parentForm, $parent_form_state);