diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/StringItem.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/StringItem.php
index cbe14fab73785de0fb38fbc505e5683ef2a4f7cd..fc7d08acf69cae868d6ce8866749d9980d4092cb 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/StringItem.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/StringItem.php
@@ -67,6 +67,9 @@ public function getConstraints() {
           '@max' => $max_length,
         ]);
       }
+      if ($this->getFieldDefinition()->getSetting('is_ascii') === TRUE) {
+        $options['charset'] = 'ASCII';
+      }
       $constraints[] = $constraint_manager->create('ComplexData', [
         'value' => [
           'Length' => $options,