diff --git a/.travis.yml b/.travis.yml
index 20a57ed..e888e79 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -15,13 +15,15 @@ matrix:
       env: PHPUNIT_TEST=1
     - php: 7.3
       env: PHPUNIT_TEST=1 PHPCS_TEST=1
+    - php: 7.4
+      env: PHPUNIT_TEST=1
 
 
 
 before_script:
   - export PATH=~/.composer/vendor/bin:$PATH
   - composer validate
-  - composer install --dev --prefer-dist
+  - composer install --prefer-dist
   - composer require --prefer-dist --no-update silverstripe/recipe-core:^4
   - composer update
   - if [[ $PHPCS_TEST ]]; then composer global require squizlabs/php_codesniffer:^3 --prefer-dist --no-interaction --no-progress --no-suggest -o; fi
diff --git a/composer.json b/composer.json
index cc8a65b..fe4b924 100644
--- a/composer.json
+++ b/composer.json
@@ -22,7 +22,7 @@
     ],
     "require": {
         "php": ">=5.6",
-        "phpunit/phpunit": "^5.7",
+        "sminnee/phpunit": "^5.7",
         "behat/behat": "^3.2",
         "behat/mink": "^1.7",
         "behat/mink-extension": "^2.1",
diff --git a/src/Extension.php b/src/Extension.php
index a864af8..24dffb1 100644
--- a/src/Extension.php
+++ b/src/Extension.php
@@ -15,6 +15,7 @@ use Symfony\Component\DependencyInjection\Loader\YamlFileLoader;
 use Symfony\Component\Config\Definition\Builder\ArrayNodeDefinition;
 use Behat\Testwork\ServiceContainer\ExtensionManager;
 use Behat\Testwork\ServiceContainer\Extension as ExtensionInterface;
+use RuntimeException;
 use Symfony\Component\DependencyInjection\Definition;
 use Symfony\Component\DependencyInjection\Reference;
 
@@ -52,8 +53,26 @@ class Extension implements ExtensionInterface
 
     public function initialize(ExtensionManager $extensionManager)
     {
-        // PHPUnit
-        require_once BASE_PATH . '/vendor/phpunit/phpunit/src/Framework/Assert/Functions.php';
+        // Find PHPUnit assertion implementations
+
+        $found = false;
+
+        $options = [
+            BASE_PATH . '/vendor/sminnee/phpunit/src/Framework/Assert/Functions.php',
+            BASE_PATH . '/vendor/phpunit/phpunit/src/Framework/Assert/Functions.php'
+        ];
+
+        foreach ($options as $file) {
+            if (file_exists($file)) {
+                require_once $file;
+                $found = true;
+                break;
+            }
+        }
+
+        if (!$found) {
+            throw new RuntimeException('Could not find PHPUnit installation');
+        }
     }
 
     public function load(ContainerBuilder $container, array $config)