diff --git a/src/testing/MD_STD_HTML_TEST.php b/src/testing/MD_STD_HTML_TEST.php
index 5b29bab..2e1c0c8 100644
--- a/src/testing/MD_STD_HTML_TEST.php
+++ b/src/testing/MD_STD_HTML_TEST.php
@@ -20,7 +20,7 @@ abstract class MD_STD_HTML_TEST extends TestCase {
      *
      * @return void
      */
-    protected static function validateHtmlString(string $input):void {
+    final protected static function validateHtmlString(string $input):void {
 
         self::assertNotEmpty($input);