diff --git a/composer.json b/composer.json
index cfe2740..2c84c22 100644
--- a/composer.json
+++ b/composer.json
@@ -24,7 +24,7 @@
         "friendsofphp/php-cs-fixer": "^3.0@dev",
         "monolog/monolog": "^2.0 || ^3.0",
         "phpspec/phpspec": "^6.1 || ^7.0",
-        "phpunit/phpunit": "^8.0 || ^9.0",
+        "phpunit/phpunit": "^9.0",
         "adlawson/vfs": "^0.12.1"
     },
     "minimum-stability": "stable",
diff --git a/composer.lock b/composer.lock
index 81c1b0b..002faab 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "ed24633a6844d6393d609656b63637c5",
+    "content-hash": "c9108b547ac432cb758490ff4b2bb355",
     "packages": [
         {
             "name": "composer/semver",
diff --git a/test/Assertion/CommandRunnerHasRunCommands.php b/test/Assertion/CommandRunnerHasRunCommands.php
index c01b896..b30753b 100644
--- a/test/Assertion/CommandRunnerHasRunCommands.php
+++ b/test/Assertion/CommandRunnerHasRunCommands.php
@@ -31,7 +31,7 @@ private function hasCommand(CommandExpectation $processPlaceholder): bool
         return false;
     }
 
-    public function evaluate($other, string $description = '', bool $returnResult = false)
+    public function evaluate($other, string $description = '', bool $returnResult = false): bool|null
     {
         /** @var CommandExpectation $command */
         foreach ($other as $command) {