diff --git a/tests/src/Functional/ToggleEditableFieldsUiTest.php b/tests/src/Functional/ToggleEditableFieldsUiTest.php
index 19846d5..f199958 100644
--- a/tests/src/Functional/ToggleEditableFieldsUiTest.php
+++ b/tests/src/Functional/ToggleEditableFieldsUiTest.php
@@ -80,7 +80,7 @@ class ToggleEditableFieldsUiTest extends BrowserTestBase {
 
     // Add a boolean field to the newly-created type.
     $label = $this->randomMachineName();
-    $field_name = Unicode::strtolower($label);
+    $field_name = mb_strtolower($label);
     $this->createBooleanField($label, $field_name, $this->nodeType1->id(), 'toggle_editable_formatter', [], [], ['label' => 'hidden', 'region' => 'content']);
 
     $this->drupalGet("admin/structure/types/manage/{$this->nodeType1->id()}/display");