diff --git a/composer.json b/composer.json
index 0c640e0..4549c01 100644
--- a/composer.json
+++ b/composer.json
@@ -16,7 +16,7 @@
     ],
     "require": {
         "php": ">=5.4",
-        "phpunit/phpunit": "^4.5 || ^5.0",
+        "phpunit/phpunit": "^4.8.35 || ^5.4.3 || ^6.0",
         "php-http/httplug": "^1.0",
         "php-http/message": "^1.0",
         "guzzlehttp/psr7": "^1.0",
diff --git a/src/HttpBaseTest.php b/src/HttpBaseTest.php
index 3c6d3b2..e697528 100644
--- a/src/HttpBaseTest.php
+++ b/src/HttpBaseTest.php
@@ -5,9 +5,10 @@
 use Http\Message\MessageFactory;
 use Http\Message\MessageFactory\GuzzleMessageFactory;
 use Nerd\CartesianProduct\CartesianProduct;
+use PHPUnit\Framework\TestCase;
 use Psr\Http\Message\ResponseInterface;
 
-abstract class HttpBaseTest extends \PHPUnit_Framework_TestCase
+abstract class HttpBaseTest extends TestCase
 {
     /**
      * @var string
diff --git a/src/HttpFeatureTest.php b/src/HttpFeatureTest.php
index 8a49f02..fa23701 100644
--- a/src/HttpFeatureTest.php
+++ b/src/HttpFeatureTest.php
@@ -5,8 +5,9 @@
 use Http\Client\HttpClient;
 use Http\Message\MessageFactory;
 use Http\Message\MessageFactory\GuzzleMessageFactory;
+use PHPUnit\Framework\TestCase;
 
-abstract class HttpFeatureTest extends \PHPUnit_Framework_TestCase
+abstract class HttpFeatureTest extends TestCase
 {
     /**
      * @var MessageFactory