diff --git a/composer.json b/composer.json
index e0bfba2..fe0e2f2 100644
--- a/composer.json
+++ b/composer.json
@@ -13,7 +13,7 @@
         "evenement/evenement": "^3.0 || ^2.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^5.0 || ^4.8.10"
+        "phpunit/phpunit": "^6.4 || ^5.7 || ^4.8.35"
     },
     "autoload": {
         "psr-4": {
diff --git a/tests/TestCase.php b/tests/TestCase.php
index 34cb790..9e090bc 100644
--- a/tests/TestCase.php
+++ b/tests/TestCase.php
@@ -2,7 +2,9 @@
 
 namespace React\Tests\HttpClient;
 
-class TestCase extends \PHPUnit_Framework_TestCase
+use PHPUnit\Framework\TestCase as BaseTestCase;
+
+class TestCase extends BaseTestCase
 {
     protected function expectCallableExactly($amount)
     {