diff --git a/core/modules/responsive_image/src/Tests/ResponsiveImageFieldDisplayTest.php b/core/modules/responsive_image/src/Tests/ResponsiveImageFieldDisplayTest.php
index 86ea80e..5a4b915 100644
--- a/core/modules/responsive_image/src/Tests/ResponsiveImageFieldDisplayTest.php
+++ b/core/modules/responsive_image/src/Tests/ResponsiveImageFieldDisplayTest.php
@@ -113,7 +113,7 @@ public function testResponsiveImageFieldFormattersLinkToNode() {
    * Test responsive image formatters linked to the file or node.
    */
   public function _testResponsiveImageFieldFormattersLink($link_type) {
-    $field_name = drupal_strtolower($this->randomMachineName());
+    $field_name = Unicode::strtolower($this->randomMachineName());
     $this->createImageField($field_name, 'article', array('uri_scheme' => 'public'));
     // Create a new node with an image attached.
     $test_image = current($this->drupalGetTestFiles('image'));