diff --git a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
index be1538785f..ed25123bf6 100644
--- a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
+++ b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
@@ -302,13 +302,16 @@ protected function assertNoField($field) {
    *
    * Raw text refers to the raw HTML that the page generated.
    *
-   * @param string $raw
+   * @param string|object $raw
    *   Raw (HTML) string to look for.
    *
    * @deprecated Scheduled for removal in Drupal 9.0.0.
    *   Use $this->assertSession()->responseContains() instead.
    */
   protected function assertRaw($raw) {
+    if (is_object($raw)) {
+      $raw = (string) $raw;
+    }
     $this->assertSession()->responseContains($raw);
   }