diff --git a/composer.json b/composer.json
index 8a1c49a3..b662d850 100644
--- a/composer.json
+++ b/composer.json
@@ -33,7 +33,7 @@
         "phpstan/phpstan": "1.12.28 || 2.1.19",
         "phpstan/phpstan-phpunit": "1.4.2 || 2.0.7",
         "phpstan/phpstan-strict-rules": "1.6.2 || 2.0.6",
-        "phpunit/phpunit": "8.5.44",
+        "phpunit/phpunit": "9.6.25",
         "rawr/phpunit-data-provider": "3.3.1",
         "rector/rector": "1.2.10 || 2.1.2",
         "rector/type-perfect": "1.0.0 || 2.1.0"
diff --git a/tests/Support/TraversableContains.php b/tests/Support/TraversableContains.php
new file mode 100644
index 00000000..4c4ece16
--- /dev/null
+++ b/tests/Support/TraversableContains.php
@@ -0,0 +1,11 @@
+<?php
+declare(strict_types=1);
+namespace Sabberworm\CSS\Tests\Support;
+
+/**
+ * Concrete subclass of the abstract PHPUnit constraint class.
+ */
+final class TraversableContains extends \PHPUnit\Framework\Constraint\TraversableContains
+{
+
+}
diff --git a/tests/Unit/Comment/CommentContainerTest.php b/tests/Unit/Comment/CommentContainerTest.php
index d0e844a4..3324bc82 100644
--- a/tests/Unit/Comment/CommentContainerTest.php
+++ b/tests/Unit/Comment/CommentContainerTest.php
@@ -5,9 +5,9 @@
 namespace Sabberworm\CSS\Tests\Unit\Comment;
 
 use PHPUnit\Framework\Constraint\LogicalAnd;
-use PHPUnit\Framework\Constraint\TraversableContains;
 use PHPUnit\Framework\TestCase;
 use Sabberworm\CSS\Comment\Comment;
+use Sabberworm\CSS\Tests\Support\TraversableContains;
 use Sabberworm\CSS\Tests\Unit\Comment\Fixtures\ConcreteCommentContainer;
 use TRegx\PhpUnit\DataProviders\DataProvider;