diff --git a/core/tests/Drupal/Tests/Component/Utility/SafeMarkupTest.php b/core/tests/Drupal/Tests/Component/Utility/SafeMarkupTest.php
index 7bda8e9..7ba686a 100644
--- a/core/tests/Drupal/Tests/Component/Utility/SafeMarkupTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/SafeMarkupTest.php
@@ -160,7 +160,7 @@ function providerCheckPlain() {
    *
    * @param string $string
    *   The string to run through SafeMarkup::format().
-   * @param string $args
+   * @param string[] $args
    *   The arguments to pass into SafeMarkup::format().
    * @param string $expected
    *   The expected result from calling the function.
@@ -169,10 +169,14 @@ function providerCheckPlain() {
    * @param bool $expected_is_safe
    *   Whether the result is expected to be safe for HTML display.
    */
-  function testFormat($string, $args, $expected, $message, $expected_is_safe) {
+  public function testFormat($string, array $args, $expected, $message, $expected_is_safe) {
     $result = SafeMarkup::format($string, $args);
     $this->assertEquals($expected, $result, $message);
     $this->assertEquals($expected_is_safe, SafeMarkup::isSafe($result), 'SafeMarkup::format correctly sets the result as safe or not safe.');
+
+    foreach ($args as $arg) {
+      $this->assertSame($arg instanceof SafeMarkupTestSafeString, SafeMarkup::isSafe($arg));
+    }
   }
 
   /**