diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..57872d0
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+/vendor/
diff --git a/composer.json b/composer.json
index f7b8b22..974fee3 100644
--- a/composer.json
+++ b/composer.json
@@ -18,6 +18,9 @@
         "silverstripe/framework": "^5",
         "silverstripe/vendor-plugin": "^1.3"
     },
+    "require-dev": {
+        "phpunit/phpunit": "^8 || ^9"
+    },
     "extra": {
         "branch-alias": {
             "dev-master": "3.x-dev"
@@ -35,5 +38,6 @@
             "SilverStripe\\TestSession\\Tests\\": "tests/"
         }
     },
-    "minimum-stability": "dev"
+    "minimum-stability": "dev",
+    "prefer-stable": true
 }
diff --git a/tests/unit/TestSessionControllerTest.php b/tests/unit/TestSessionControllerTest.php
index 03e137c..f63f070 100644
--- a/tests/unit/TestSessionControllerTest.php
+++ b/tests/unit/TestSessionControllerTest.php
@@ -4,6 +4,7 @@ namespace SilverStripe\TestSession\Tests\Unit;
 
 use DateTime;
 use LogicException;
+use PHPUnit\Framework\MockObject\MockObject;
 use SilverStripe\Control\Director;
 use SilverStripe\Control\Email\Mailer;
 use SilverStripe\Control\HTTPRequest;
@@ -22,13 +23,13 @@ use SilverStripe\Core\Config\Config;
 class TestSessionControllerTest extends SapphireTest
 {
     /**
-     * @var TestSessionEnvironment|PHPUnit_Framework_MockObject_MockObject
+     * @var TestSessionEnvironment|MockObject
      */
     private $testSessionEnvironment;
 
     protected $usesDatabase = true;
 
-    protected function setUp()
+    protected function setUp() : void
     {
         parent::setUp();
         Injector::inst()->unregisterNamedObject(TestSessionEnvironment::class);
diff --git a/tests/unit/TestSessionEnvironmentTest.php b/tests/unit/TestSessionEnvironmentTest.php
index cd85f95..6db424f 100644
--- a/tests/unit/TestSessionEnvironmentTest.php
+++ b/tests/unit/TestSessionEnvironmentTest.php
@@ -18,7 +18,7 @@ use stdClass;
 class TestSessionEnvironmentTest extends SapphireTest
 {
 
-    protected function setUp()
+    protected function setUp() : void
     {
         parent::setUp();
         Injector::inst()->unregisterNamedObject(TestSessionEnvironment::class);
diff --git a/tests/unit/TestSessionHTTPMiddlewareTest.php b/tests/unit/TestSessionHTTPMiddlewareTest.php
index 325de56..bd6d5ae 100644
--- a/tests/unit/TestSessionHTTPMiddlewareTest.php
+++ b/tests/unit/TestSessionHTTPMiddlewareTest.php
@@ -23,7 +23,7 @@ class TestSessionHTTPMiddlewareTest extends SapphireTest
 
     protected $usesDatabase = true;
 
-    protected function setUp()
+    protected function setUp() : void
     {
         parent::setUp();
         Injector::inst()->registerService(