diff --git a/.travis.yml b/.travis.yml
index 1b479db43..c0473a49a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,6 +8,7 @@ php:
   - 7.1
   - 7.2
   - 7.3
+  - nightly
   - hhvm
 
 install: travis_retry composer install --no-interaction --prefer-source
@@ -18,6 +19,7 @@ script:
 
 matrix:
   allow_failures:
+    - php: nightly
     - php: hhvm
     - php: 7.3
   fast_finish: true
diff --git a/composer.json b/composer.json
index d351a924e..01135e5ec 100644
--- a/composer.json
+++ b/composer.json
@@ -33,7 +33,7 @@
         "phenx/php-svg-lib": "0.3.*"
     },
     "require-dev": {
-        "phpunit/phpunit": "4.8.*",
+        "phpunit/phpunit": "^4.8|^5.5|^6.5",
         "squizlabs/php_codesniffer": "2.*"
     },
     "extra": {
diff --git a/tests/Dompdf/Tests/AutoloaderTest.php b/tests/Dompdf/Tests/AutoloaderTest.php
index 91ee9205b..bd77e5bcf 100644
--- a/tests/Dompdf/Tests/AutoloaderTest.php
+++ b/tests/Dompdf/Tests/AutoloaderTest.php
@@ -1,10 +1,10 @@
 <?php
 namespace Dompdf\Tests;
 
-use PHPUnit_Framework_TestCase;
+use PHPUnit\Framework\TestCase;
 use Dompdf\Autoloader;
 
-class AutoloaderTest extends PHPUnit_Framework_TestCase
+class AutoloaderTest extends TestCase
 {
     public function testAutoload()
     {
diff --git a/tests/Dompdf/Tests/DompdfTest.php b/tests/Dompdf/Tests/DompdfTest.php
index a74665b64..2929e4afe 100644
--- a/tests/Dompdf/Tests/DompdfTest.php
+++ b/tests/Dompdf/Tests/DompdfTest.php
@@ -3,12 +3,12 @@
 
 use Dompdf\Frame\FrameTree;
 use Dompdf\Options;
-use PHPUnit_Framework_TestCase;
+use PHPUnit\Framework\TestCase;
 use Dompdf\Dompdf;
 use Dompdf\Css\Stylesheet;
 use DOMDocument;
 
-class DompdfTest extends PHPUnit_Framework_TestCase
+class DompdfTest extends TestCase
 {
     public function testConstructor()
     {
diff --git a/tests/Dompdf/Tests/HelpersTest.php b/tests/Dompdf/Tests/HelpersTest.php
index da88db504..0440f50e9 100644
--- a/tests/Dompdf/Tests/HelpersTest.php
+++ b/tests/Dompdf/Tests/HelpersTest.php
@@ -2,9 +2,9 @@
 namespace Dompdf\Tests;
 
 use Dompdf\Helpers;
-use PHPUnit_Framework_TestCase;
+use PHPUnit\Framework\TestCase;
 
-class HelpersTest extends PHPUnit_Framework_TestCase
+class HelpersTest extends TestCase
 {
     public function testParseDataUriBase64Image()
     {
diff --git a/tests/Dompdf/Tests/OptionsTest.php b/tests/Dompdf/Tests/OptionsTest.php
index 659441fd6..ad7c74f84 100644
--- a/tests/Dompdf/Tests/OptionsTest.php
+++ b/tests/Dompdf/Tests/OptionsTest.php
@@ -2,9 +2,9 @@
 namespace Dompdf\Tests;
 
 use Dompdf\Options;
-use PHPUnit_Framework_TestCase;
+use PHPUnit\Framework\TestCase;
 
-class OptionsTest extends PHPUnit_Framework_TestCase
+class OptionsTest extends TestCase
 {
     public function testConstructor()
     {