diff --git a/test/Unit/FactoryTest.php b/test/Unit/FactoryTest.php
index df66428d..41cb9130 100644
--- a/test/Unit/FactoryTest.php
+++ b/test/Unit/FactoryTest.php
@@ -14,7 +14,6 @@
 namespace Ergebnis\PhpCsFixer\Config\Test\Unit;
 
 use Ergebnis\PhpCsFixer\Config;
-use PhpCsFixer\ConfigInterface;
 use PHPUnit\Framework;
 
 /**
@@ -95,7 +94,6 @@ public function testFromRuleSetCreatesConfig(int $targetPhpVersion): void
 
         $config = Config\Factory::fromRuleSet($ruleSet->reveal());
 
-        self::assertInstanceOf(ConfigInterface::class, $config);
         self::assertTrue($config->getUsingCache());
         self::assertTrue($config->getRiskyAllowed());
         self::assertSame($rules, $config->getRules());
@@ -155,7 +153,6 @@ public function testFromRuleSetCreatesConfigWithOverrideRules(): void
             $overrideRules
         );
 
-        self::assertInstanceOf(ConfigInterface::class, $config);
         self::assertTrue($config->getUsingCache());
         self::assertTrue($config->getRiskyAllowed());
         self::assertSame(\array_merge($rules, $overrideRules), $config->getRules());
diff --git a/test/Unit/RuleSet/AbstractRuleSetTestCase.php b/test/Unit/RuleSet/AbstractRuleSetTestCase.php
index 62530e06..3ee636c5 100644
--- a/test/Unit/RuleSet/AbstractRuleSetTestCase.php
+++ b/test/Unit/RuleSet/AbstractRuleSetTestCase.php
@@ -227,7 +227,7 @@ private function builtInFixers(): array
             $fixerFactory = FixerFactory::create();
             $fixerFactory->registerBuiltInFixers();
 
-            $builtInFixers = \array_map(static function (Fixer\FixerInterface $fixer) {
+            $builtInFixers = \array_map(static function (Fixer\FixerInterface $fixer): string {
                 return $fixer->getName();
             }, $fixerFactory->getFixers());
         }
@@ -242,7 +242,7 @@ private function configuredFixers(): array
          *
          * @see https://github.com/FriendsOfPHP/PHP-CS-Fixer/pull/2361
          */
-        $rules = \array_map(static function () {
+        $rules = \array_map(static function (): bool {
             return true;
         }, self::createRuleSet()->rules());