diff --git a/src/Cache/FileStore.php b/src/Cache/FileStore.php
index 9713fde..41efd29 100644
--- a/src/Cache/FileStore.php
+++ b/src/Cache/FileStore.php
@@ -15,7 +15,7 @@ class FileStore implements CacheInterface
 
     private readonly string $directory;
 
-    public function __construct(string $directory = null)
+    public function __construct(?string $directory = null)
     {
         $this->directory = $directory ?? (sys_get_temp_dir().DIRECTORY_SEPARATOR.self::CACHE_FOLDER_NAME); // @pest-mutate-ignore
 
@@ -29,7 +29,7 @@ public function get(string $key, mixed $default = null): mixed
         return $this->getPayload($key) ?? $default;
     }
 
-    public function set(string $key, mixed $value, DateInterval|int $ttl = null): bool
+    public function set(string $key, mixed $value, DateInterval|int|null $ttl = null): bool
     {
         $payload = serialize($value);
 
@@ -77,7 +77,7 @@ public function getMultiple(iterable $keys, mixed $default = null): iterable
     /**
      * @param  iterable<string, mixed>  $values
      */
-    public function setMultiple(iterable $values, DateInterval|int $ttl = null): bool // @phpstan-ignore-line
+    public function setMultiple(iterable $values, DateInterval|int|null $ttl = null): bool // @phpstan-ignore-line
     {
         $result = true;
 
diff --git a/src/Cache/NullStore.php b/src/Cache/NullStore.php
index 0010924..7d574ec 100644
--- a/src/Cache/NullStore.php
+++ b/src/Cache/NullStore.php
@@ -14,7 +14,7 @@ public function get(string $key, mixed $default = null): mixed
         return $default;
     }
 
-    public function set(string $key, mixed $value, DateInterval|int $ttl = null): bool
+    public function set(string $key, mixed $value, DateInterval|int|null $ttl = null): bool
     {
         return true;
     }
@@ -42,7 +42,7 @@ public function getMultiple(iterable $keys, mixed $default = null): iterable
     /**
      * @param  iterable<string, mixed>  $values
      */
-    public function setMultiple(iterable $values, DateInterval|int $ttl = null): bool // @phpstan-ignore-line
+    public function setMultiple(iterable $values, DateInterval|int|null $ttl = null): bool // @phpstan-ignore-line
     {
         return true;
     }
diff --git a/src/Contracts/Configuration.php b/src/Contracts/Configuration.php
index a230fa6..bb39842 100644
--- a/src/Contracts/Configuration.php
+++ b/src/Contracts/Configuration.php
@@ -26,7 +26,7 @@ public function mutator(array|string ...$mutators): self;
      */
     public function except(array|string ...$mutators): self;
 
-    public function min(float $minScore, bool $failOnZeroMutations = null): self;
+    public function min(float $minScore, ?bool $failOnZeroMutations = null): self;
 
     public function ignoreMinScoreOnZeroMutations(bool $ignore = true): self;
 
@@ -34,7 +34,7 @@ public function coveredOnly(bool $coveredOnly = true): self;
 
     public function parallel(bool $parallel = true): self;
 
-    public function processes(int $processes = null): self;
+    public function processes(?int $processes = null): self;
 
     public function profile(bool $profile = true): self;
 
diff --git a/src/Decorators/TestCallDecorator.php b/src/Decorators/TestCallDecorator.php
index ba7078d..797497f 100644
--- a/src/Decorators/TestCallDecorator.php
+++ b/src/Decorators/TestCallDecorator.php
@@ -65,7 +65,7 @@ public function coveredOnly(bool $coveredOnly = true): self
         return $this;
     }
 
-    public function min(float $minScore, bool $failOnZeroMutations = null): self
+    public function min(float $minScore, ?bool $failOnZeroMutations = null): self
     {
         $this->configuration->min($minScore);
         if ($failOnZeroMutations !== null) {
@@ -129,7 +129,7 @@ public function parallel(bool $parallel = true): self
         return $this;
     }
 
-    public function processes(int $processes = null): Configuration
+    public function processes(?int $processes = null): Configuration
     {
         $this->configuration->processes($processes);
 
diff --git a/src/Mutation.php b/src/Mutation.php
index 548755d..a37c1e9 100644
--- a/src/Mutation.php
+++ b/src/Mutation.php
@@ -18,12 +18,12 @@
 class Mutation
 {
     private const TMP_FOLDER = __DIR__
-    .DIRECTORY_SEPARATOR
-    .'..'
-    .DIRECTORY_SEPARATOR
-    .'.temp'
-    .DIRECTORY_SEPARATOR
-    .'mutations';
+        .DIRECTORY_SEPARATOR
+        .'..'
+        .DIRECTORY_SEPARATOR
+        .'.temp'
+        .DIRECTORY_SEPARATOR
+        .'mutations';
 
     private const DIFF_SEPARATOR = '--- Expected'.PHP_EOL.'+++ Actual'.PHP_EOL.'@@ @@'.PHP_EOL;
 
diff --git a/src/MutationTest.php b/src/MutationTest.php
index 49fa355..06765d8 100644
--- a/src/MutationTest.php
+++ b/src/MutationTest.php
@@ -47,7 +47,7 @@ public function updateResult(MutationTestResult $result): void
      * @param  array<string, array<int, array<int, string>>>  $coveredLines
      * @param  array<int, string>  $originalArguments
      */
-    public function start(array $coveredLines, Configuration $configuration, array $originalArguments, int $processId = null): bool
+    public function start(array $coveredLines, Configuration $configuration, array $originalArguments, ?int $processId = null): bool
     {
         // TODO: we should pass the tests to run in another way, maybe via cache, mutation or env variable
         $filters = [];
diff --git a/src/Repositories/ConfigurationRepository.php b/src/Repositories/ConfigurationRepository.php
index fdf4f5c..265a03d 100644
--- a/src/Repositories/ConfigurationRepository.php
+++ b/src/Repositories/ConfigurationRepository.php
@@ -53,7 +53,7 @@ public function setProfile(string $profile): void
         $this->profile = $profile;
     }
 
-    public function globalConfiguration(string $name = null): GlobalConfiguration
+    public function globalConfiguration(?string $name = null): GlobalConfiguration
     {
         $name ??= $this->profile;
 
@@ -64,7 +64,7 @@ public function globalConfiguration(string $name = null): GlobalConfiguration
         return $this->globalConfigurations[$name];
     }
 
-    public function fakeTestConfiguration(string $name = null): TestConfiguration
+    public function fakeTestConfiguration(?string $name = null): TestConfiguration
     {
         $name ??= $this->profile;
 
diff --git a/src/Support/Configuration/AbstractConfiguration.php b/src/Support/Configuration/AbstractConfiguration.php
index 3025083..70ead13 100644
--- a/src/Support/Configuration/AbstractConfiguration.php
+++ b/src/Support/Configuration/AbstractConfiguration.php
@@ -100,7 +100,7 @@ public function except(array|string ...$mutators): self
         return $this;
     }
 
-    public function min(float $minScore, bool $failOnZeroMutations = null): self
+    public function min(float $minScore, ?bool $failOnZeroMutations = null): self
     {
         $this->minScore = $minScore;
 
@@ -132,7 +132,7 @@ public function parallel(bool $parallel = true): self
         return $this;
     }
 
-    public function processes(int $processes = null): self
+    public function processes(?int $processes = null): self
     {
         $this->processes = $processes;