diff --git a/core/tests/Drupal/Tests/Listeners/SimpletestUiPrinter7.php b/core/tests/Drupal/Tests/Listeners/SimpletestUiPrinter7.php
index fca0b5952c..b5720620d1 100644
--- a/core/tests/Drupal/Tests/Listeners/SimpletestUiPrinter7.php
+++ b/core/tests/Drupal/Tests/Listeners/SimpletestUiPrinter7.php
@@ -12,7 +12,7 @@ class SimpletestUiPrinter7 extends HtmlOutputPrinter {
   /**
    * {@inheritdoc}
    */
-  public function write($buffer): void {
+  public function write(string $buffer): void {
     $buffer = Html::escape($buffer);
     // Turn HTML output URLs into clickable link <a> tags.
     $url_pattern = '@https?://[^\s]+@';