diff --git a/composer.json b/composer.json
index 63c7d4a..5f6a880 100644
--- a/composer.json
+++ b/composer.json
@@ -26,7 +26,7 @@
         "opis/closure": "Allow to serialize PHP closures"
     },
     "require-dev": {
-        "phpunit/phpunit": ">=6.0 <10.0"
+        "phpunit/phpunit": ">=6.0 <11.0"
     },
     "autoload": {
         "psr-4": {
diff --git a/tests/ClosureSerializer/ClosureSerializerManagerTest.php b/tests/ClosureSerializer/ClosureSerializerManagerTest.php
index b33dfef..b52abf4 100644
--- a/tests/ClosureSerializer/ClosureSerializerManagerTest.php
+++ b/tests/ClosureSerializer/ClosureSerializerManagerTest.php
@@ -8,6 +8,13 @@
 
 class ClosureSerializerManagerTest extends TestCase
 {
+    public function setUp(): void
+    {
+        if (! class_exists(\SuperClosure\SerializerInterface::class)) {
+            $this->markTestSkipped('Missing jeremeamia/superclosure to run this test');
+        }
+    }
+
     public function testAddSerializer() {
         $manager = new ClosureSerializerManager();
         $this->assertEmpty($manager->getSerializer('foo'));
diff --git a/tests/ClosureSerializer/OpisClosureSerializerTest.php b/tests/ClosureSerializer/OpisClosureSerializerTest.php
index 5950c23..54708a1 100644
--- a/tests/ClosureSerializer/OpisClosureSerializerTest.php
+++ b/tests/ClosureSerializer/OpisClosureSerializerTest.php
@@ -7,6 +7,13 @@
 
 class OpisClosureSerializerTest extends TestCase
 {
+    public function setUp(): void
+    {
+        if (! class_exists(\Opis\Closure\SerializableClosure::class)) {
+            $this->markTestSkipped('Missing opis/closure to run this test');
+        }
+    }
+
     public function testSerialize() {
         $closure = function() {
             return 'foo';
diff --git a/tests/ClosureSerializer/SuperClosureSerializerTest.php b/tests/ClosureSerializer/SuperClosureSerializerTest.php
index 7ba40c3..092220e 100644
--- a/tests/ClosureSerializer/SuperClosureSerializerTest.php
+++ b/tests/ClosureSerializer/SuperClosureSerializerTest.php
@@ -7,6 +7,13 @@
 
 class SuperClosureSerializerTest extends TestCase
 {
+    public function setUp(): void
+    {
+        if (! class_exists(\SuperClosure\SerializerInterface::class)) {
+            $this->markTestSkipped('Missing jeremeamia/superclosure to run this test');
+        }
+    }
+
     public function testSerialize() {
         $closure = function() {
             return 'foo';
diff --git a/tests/JsonSerializerTest.php b/tests/JsonSerializerTest.php
index 15fd8df..afa4512 100644
--- a/tests/JsonSerializerTest.php
+++ b/tests/JsonSerializerTest.php
@@ -81,10 +81,8 @@ public function testUnserializeScalar($scalar, $jsoned)
 
     /**
      * List of scalar data
-     *
-     * @return array
      */
-    public function scalarData()
+    public static function scalarData(): array
     {
         return array(
             array('testing', '"testing"'),
@@ -156,10 +154,8 @@ public function testUnserializeArrayNoObject($array, $jsoned)
 
     /**
      * List of array data
-     *
-     * @return array
      */
-    public function arrayNoObjectData()
+    public static function arrayNoObjectData(): array
     {
         return array(
             array(array(1, 2, 3), '[1,2,3]'),