diff --git a/core/drupalci.yml b/core/drupalci.yml
index 893046afce..7621541666 100644
--- a/core/drupalci.yml
+++ b/core/drupalci.yml
@@ -43,20 +43,20 @@ build:
         testgroups: '--all'
         suppress-deprecations: false
         halt-on-fail: false
-#      run_tests.functional:
-#        types: 'PHPUnit-Functional'
-#        testgroups: '--all'
-#        suppress-deprecations: false
-#        halt-on-fail: false
-#      run_tests.javascript:
-#        concurrency: 15
-#        types: 'PHPUnit-FunctionalJavascript'
-#        testgroups: '--all'
-#        suppress-deprecations: false
-#        halt-on-fail: false
-#      # Run nightwatch testing.
-#      # @see https://www.drupal.org/project/drupal/issues/2869825
-#      nightwatchjs:
+      run_tests.functional:
+        types: 'PHPUnit-Functional'
+        testgroups: '--all'
+        suppress-deprecations: false
+        halt-on-fail: false
+      run_tests.javascript:
+        concurrency: 15
+        types: 'PHPUnit-FunctionalJavascript'
+        testgroups: '--all'
+        suppress-deprecations: false
+        halt-on-fail: false
+      # Run nightwatch testing.
+      # @see https://www.drupal.org/project/drupal/issues/2869825
+      nightwatchjs:
 #      container_command.drupal_project_templates:
 #        commands:
 #          - "sudo -u www-data ${SOURCE_DIR}/core/tests/scripts/test_composer_project_templates.sh"
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 34508bcce0..4b666f5e79 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1128,7 +1128,8 @@ function template_preprocess_item_list(&$variables) {
             // \Drupal\Core\Render\Element::children(), which cannot be used
             // here, since it triggers an error on string values.
             foreach ($child as $child_key => $child_value) {
-              if (is_string($child_key) && $child_key[0] !== '#') {
+              $child_key = (string) $child_key;
+              if ($child_key[0] !== '#') {
                 $child['#items'][$child_key] = $child_value;
                 unset($child[$child_key]);
               }
diff --git a/core/lib/Drupal/Core/Render/Element.php b/core/lib/Drupal/Core/Render/Element.php
index 02c3b038dd..5322deba91 100644
--- a/core/lib/Drupal/Core/Render/Element.php
+++ b/core/lib/Drupal/Core/Render/Element.php
@@ -78,7 +78,8 @@ public static function children(array &$elements, $sort = FALSE) {
     $i = 0;
     $sortable = FALSE;
     foreach ($elements as $key => $value) {
-      if ($key === '' || (is_string($key) && $key[0] !== '#')) {
+      $key = (string) $key;
+      if ($key === '' || $key[0] !== '#') {
         if (is_array($value)) {
           if (isset($value['#weight'])) {
             $weight = $value['#weight'];
diff --git a/core/lib/Drupal/Core/Security/RequestSanitizer.php b/core/lib/Drupal/Core/Security/RequestSanitizer.php
index e1626ed383..87fc93c623 100644
--- a/core/lib/Drupal/Core/Security/RequestSanitizer.php
+++ b/core/lib/Drupal/Core/Security/RequestSanitizer.php
@@ -153,6 +153,7 @@ protected static function checkDestination($destination, array $whitelist) {
   protected static function stripDangerousValues($input, array $whitelist, array &$sanitized_keys) {
     if (is_array($input)) {
       foreach ($input as $key => $value) {
+        $key = (string) $key;
         if ($key !== '' && $key[0] === '#' && !in_array($key, $whitelist, TRUE)) {
           unset($input[$key]);
           $sanitized_keys[] = $key;