diff --git a/core/lib/Drupal/Core/Form/FormValidator.php b/core/lib/Drupal/Core/Form/FormValidator.php
index d7d70f3503..77515c10d1 100644
--- a/core/lib/Drupal/Core/Form/FormValidator.php
+++ b/core/lib/Drupal/Core/Form/FormValidator.php
@@ -345,7 +345,7 @@ protected function performRequiredValidation(&$elements, FormStateInterface &$fo
         $value = in_array($elements['#type'], ['checkboxes', 'tableselect']) ? array_keys($elements['#value']) : $elements['#value'];
         foreach ($value as $v) {
           if (!isset($options[$v])) {
-            $form_state->setError($elements, $this->t('An illegal choice has been detected. Please contact the site administrator.'));
+            $form_state->setError($elements, $this->t('An invalid choice has been detected. Please, contact the site administrator.'));
             $this->logger->error('Illegal choice %choice in %name element.', ['%choice' => $v, '%name' => empty($elements['#title']) ? $elements['#parents'][0] : $elements['#title']]);
           }
         }
@@ -364,7 +364,7 @@ protected function performRequiredValidation(&$elements, FormStateInterface &$fo
         $form_state->setValueForElement($elements, NULL);
       }
       elseif (!isset($options[$elements['#value']])) {
-        $form_state->setError($elements, $this->t('An illegal choice has been detected. Please contact the site administrator.'));
+        $form_state->setError($elements, $this->t('An invalid choice has been detected. Please, contact the site administrator.'));
         $this->logger->error('Illegal choice %choice in %name element.', ['%choice' => $elements['#value'], '%name' => empty($elements['#title']) ? $elements['#parents'][0] : $elements['#title']]);
       }
     }
diff --git a/core/modules/block/tests/src/FunctionalJavascript/BlockAddTest.php b/core/modules/block/tests/src/FunctionalJavascript/BlockAddTest.php
index 17e020d8cc..9bfb24efe5 100644
--- a/core/modules/block/tests/src/FunctionalJavascript/BlockAddTest.php
+++ b/core/modules/block/tests/src/FunctionalJavascript/BlockAddTest.php
@@ -44,7 +44,7 @@ public function testBlockAddThemeSelector() {
     // Switch to a theme that doesn't contain the region selected above.
     $assert_session->selectExists('Theme')->selectOption('stark');
     $assert_session->assertWaitOnAjaxRequest();
-    $assert_session->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
+    $assert_session->pageTextNotContains('An invalid choice has been detected. Please, contact the site administrator..');
     $assert_session->optionExists('Region', '- Select -');
   }
 
diff --git a/core/modules/comment/tests/src/Functional/CommentFieldsTest.php b/core/modules/comment/tests/src/Functional/CommentFieldsTest.php
index 14b538bd4e..e691c3c641 100644
--- a/core/modules/comment/tests/src/Functional/CommentFieldsTest.php
+++ b/core/modules/comment/tests/src/Functional/CommentFieldsTest.php
@@ -165,7 +165,7 @@ public function testCommentFieldCreate() {
     $this->drupalGet('admin/config/people/accounts/fields/user.user.field_user_comment/storage');
     $this->submitForm($edit, 'Save field settings');
     // We should get an error message.
-    $this->assertSession()->pageTextContains('An illegal choice has been detected. Please contact the site administrator.');
+    $this->assertSession()->pageTextContains('An invalid choice has been detected. Please, contact the site administrator.');
 
     // Create a comment type for users.
     $bundle = CommentType::create([
@@ -183,7 +183,7 @@ public function testCommentFieldCreate() {
     $this->drupalGet('admin/config/people/accounts/fields/user.user.field_user_comment/storage');
     $this->submitForm($edit, 'Save field settings');
     // We shouldn't get an error message.
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
+    $this->assertSession()->pageTextNotContains('An invalid choice has been detected. Please, contact the site administrator..');
   }
 
   /**
diff --git a/core/modules/system/tests/src/Functional/Form/FormTest.php b/core/modules/system/tests/src/Functional/Form/FormTest.php
index 5599bf9dff..5ab77a4860 100644
--- a/core/modules/system/tests/src/Functional/Form/FormTest.php
+++ b/core/modules/system/tests/src/Functional/Form/FormTest.php
@@ -886,7 +886,7 @@ public function testInputForgery() {
     // an input forgery.
     // @see \Drupal\form_test\Form\FormTestInputForgeryForm::postRender
     $this->submitForm(['checkboxes[one]' => TRUE, 'checkboxes[two]' => TRUE], 'Submit');
-    $this->assertSession()->pageTextContains('An illegal choice has been detected.');
+    $this->assertSession()->pageTextContains('An invalid choice has been detected.');
   }
 
   /**
diff --git a/core/modules/system/tests/src/Functional/Form/ValidationTest.php b/core/modules/system/tests/src/Functional/Form/ValidationTest.php
index 0738c34b23..c84e062cdb 100644
--- a/core/modules/system/tests/src/Functional/Form/ValidationTest.php
+++ b/core/modules/system/tests/src/Functional/Form/ValidationTest.php
@@ -235,7 +235,7 @@ public function testCustomRequiredError() {
         $this->assertSession()->pageTextContains((string) $form[$key]['#form_test_required_error']);
       }
     }
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
+    $this->assertSession()->pageTextNotContains('An invalid choice has been detected. Please, contact the site administrator.');
 
     // Verify that no custom validation error appears with valid values.
     $edit = [
@@ -256,7 +256,7 @@ public function testCustomRequiredError() {
         $this->assertSession()->pageTextNotContains((string) $form[$key]['#form_test_required_error']);
       }
     }
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
+    $this->assertSession()->pageTextNotContains('An invalid choice has been detected. Please, contact the site administrator.');
   }
 
 }
diff --git a/core/modules/views/tests/src/Functional/Plugin/ExposedFormCheckboxesTest.php b/core/modules/views/tests/src/Functional/Plugin/ExposedFormCheckboxesTest.php
index e4b1b5417d..792a0fc71e 100644
--- a/core/modules/views/tests/src/Functional/Plugin/ExposedFormCheckboxesTest.php
+++ b/core/modules/views/tests/src/Functional/Plugin/ExposedFormCheckboxesTest.php
@@ -109,7 +109,7 @@ public function testExposedFormRenderCheckboxes() {
     // checked.
     $this->clickLink('Page 2');
     $this->assertSession()->elementsCount('xpath', "//div[contains(@class, 'views-row')]", 1);
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
+    $this->assertSession()->pageTextNotContains('An invalid choice has been detected. Please, contact the site administrator.');
   }
 
   /**
@@ -158,14 +158,14 @@ public function testExposedIsAllOfFilter() {
     // All rows are displayed by default on the first page when no options are
     // checked.
     $this->assertSession()->elementsCount('xpath', "//div[contains(@class, 'views-row')]", 8);
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
+    $this->assertSession()->pageTextNotContains('An invalid choice has been detected. Please, contact the site administrator.');
 
     // Select one option and ensure we still have results.
     $tid = $this->terms[0]->id();
     $this->submitForm(["tid[$tid]" => $tid], 'Apply');
     // Ensure only nodes tagged with $tid are displayed.
     $this->assertSession()->elementsCount('xpath', "//div[contains(@class, 'views-row')]", 2);
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected. Please contact the site administrator.');
+    $this->assertSession()->pageTextNotContains('An invalid choice has been detected. Please, contact the site administrator.');
   }
 
 }
diff --git a/core/modules/views/tests/src/Functional/Plugin/FilterTest.php b/core/modules/views/tests/src/Functional/Plugin/FilterTest.php
index b153916439..8a4d74cdfd 100644
--- a/core/modules/views/tests/src/Functional/Plugin/FilterTest.php
+++ b/core/modules/views/tests/src/Functional/Plugin/FilterTest.php
@@ -173,7 +173,7 @@ public function testInOperatorSelectAllOptions() {
     $this->drupalGet('admin/structure/views/view/test_filter_in_operator_ui/edit/default');
     $this->submitForm([], 'Save');
     $this->submitForm([], 'Update preview');
-    $this->assertSession()->pageTextNotContains('An illegal choice has been detected.');
+    $this->assertSession()->pageTextNotContains('An invalid choice has been detected.');
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/Core/Form/FormValidatorTest.php b/core/tests/Drupal/Tests/Core/Form/FormValidatorTest.php
index eccceecc59..b3f324fe34 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormValidatorTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormValidatorTest.php
@@ -424,7 +424,7 @@ public function providerTestPerformRequiredValidation() {
           '#value' => 'baz',
           '#multiple' => FALSE,
         ],
-        'An illegal choice has been detected. Please contact the site administrator.',
+        'An invalid choice has been detected. Please, contact the site administrator.',
         TRUE,
       ],
       [
@@ -437,7 +437,7 @@ public function providerTestPerformRequiredValidation() {
           '#value' => ['baz'],
           '#multiple' => TRUE,
         ],
-        'An illegal choice has been detected. Please contact the site administrator.',
+        'An invalid choice has been detected. Please, contact the site administrator.',
         TRUE,
       ],
       [
@@ -450,7 +450,7 @@ public function providerTestPerformRequiredValidation() {
           '#value' => ['baz'],
           '#multiple' => TRUE,
         ],
-        'An illegal choice has been detected. Please contact the site administrator.',
+        'An invalid choice has been detected. Please, contact the site administrator.',
         TRUE,
       ],
       [