diff --git a/core/modules/responsive_image/responsive_image.module b/core/modules/responsive_image/responsive_image.module
index 08e403f..15ad0a0 100644
--- a/core/modules/responsive_image/responsive_image.module
+++ b/core/modules/responsive_image/responsive_image.module
@@ -310,7 +310,7 @@ function responsive_image_build_source_tags(ImageInterface $image, $variables) {
             $derivative_mime_type = responsive_image_get_mime_type($mapping_definition['image_style'], $extension);
             $derivative_mime_types[] = $derivative_mime_type;
             // Add the image source with its multiplier.
-            $srcset[floatval(drupal_substr($multiplier, 0, -1))] = array(
+            $srcset[floatval(Unicode::substr($multiplier, 0, -1))] = array(
               'uri' => _responsive_image_image_style_url($mapping_definition['image_style'], $image->getSource()),
               'multiplier' => $multiplier,
             );