diff --git a/core/tests/Drupal/TestTools/PhpUnitCompatibility/PhpUnit6/TestCompatibilityTrait.php b/core/tests/Drupal/TestTools/PhpUnitCompatibility/PhpUnit6/TestCompatibilityTrait.php
index c08bfb1a5c..710cc2d297 100644
--- a/core/tests/Drupal/TestTools/PhpUnitCompatibility/PhpUnit6/TestCompatibilityTrait.php
+++ b/core/tests/Drupal/TestTools/PhpUnitCompatibility/PhpUnit6/TestCompatibilityTrait.php
@@ -71,28 +71,28 @@ public static function assertNotEqualsCanonicalizing($expected, $actual, $messag
    * Forward compatibility for assertStringContainsString.
    */
   public static function assertStringContainsString($needle, $haystack, $message = '') {
-    static::assertContains($needle, $haystack, $message);
+    static::assertContains((string) $needle, (string) $haystack, $message);
   }
 
   /**
    * Forward compatibility for assertStringContainsStringIgnoringCase.
    */
   public static function assertStringContainsStringIgnoringCase($needle, $haystack, $message = '') {
-    static::assertContains($needle, $haystack, $message, TRUE);
+    static::assertContains((string) $needle, (string) $haystack, $message, TRUE);
   }
 
   /**
    * Forward compatibility for assertStringNotContainsString.
    */
   public static function assertStringNotContainsString($needle, $haystack, $message = '') {
-    static::assertNotContains($needle, $haystack, $message);
+    static::assertNotContains((string) $needle, (string) $haystack, $message);
   }
 
   /**
    * Forward compatibility for assertStringNotContainsStringIgnoringCase.
    */
   public static function assertStringNotContainsStringIgnoringCase($needle, $haystack, $message = '') {
-    static::assertNotContains($needle, $haystack, $message, TRUE);
+    static::assertNotContains((string) $needle, (string) $haystack, $message, TRUE);
   }
 
   /**
diff --git a/core/tests/Drupal/Tests/PhpunitCompatibilityTraitTest.php b/core/tests/Drupal/Tests/PhpunitCompatibilityTraitTest.php
index c909c3ccc7..1a60814507 100644
--- a/core/tests/Drupal/Tests/PhpunitCompatibilityTraitTest.php
+++ b/core/tests/Drupal/Tests/PhpunitCompatibilityTraitTest.php
@@ -2,6 +2,8 @@
 
 namespace Drupal\Tests;
 
+use Drupal\Component\Render\FormattableMarkup;
+
 /**
  * Tests the PHPUnit forward compatibility trait.
  *
@@ -44,8 +46,8 @@ public function testSetExpectedException() {
    * @covers ::assertStringNotContainsStringIgnoringCase
    */
   public function testAssertStringContainsString() {
-    $this->assertStringContainsString("bingo", "foobarbingobongo");
-    $this->assertStringContainsStringIgnoringCase("bingo", "foobarBiNgObongo");
+    $this->assertStringContainsString("bingo", new FormattableMarkup("foobarbingobongo", []));
+    $this->assertStringContainsStringIgnoringCase(new FormattableMarkup("bingo", []), "foobarBiNgObongo");
     $this->assertStringNotContainsString("buzzer", "BUZZERbingobongo");
     $this->assertStringNotContainsStringIgnoringCase("buzzer", "foobarBiNgObongo");
   }