diff --git a/Command/DotenvDumpCommand.php b/Command/DotenvDumpCommand.php
index 4411054..5b3b84e 100644
--- a/Command/DotenvDumpCommand.php
+++ b/Command/DotenvDumpCommand.php
@@ -33,7 +33,7 @@ final class DotenvDumpCommand extends Command
     private $projectDir;
     private $defaultEnv;
 
-    public function __construct(string $projectDir, string $defaultEnv = null)
+    public function __construct(string $projectDir, ?string $defaultEnv = null)
     {
         $this->projectDir = $projectDir;
         $this->defaultEnv = $defaultEnv;
diff --git a/Dotenv.php b/Dotenv.php
index 685df57..b454452 100644
--- a/Dotenv.php
+++ b/Dotenv.php
@@ -107,7 +107,7 @@ public function load(string $path, string ...$extraPaths): void
      * @throws FormatException when a file has a syntax error
      * @throws PathException   when a file does not exist or is not readable
      */
-    public function loadEnv(string $path, string $envKey = null, string $defaultEnv = 'dev', array $testEnvs = ['test'], bool $overrideExistingVars = false): void
+    public function loadEnv(string $path, ?string $envKey = null, string $defaultEnv = 'dev', array $testEnvs = ['test'], bool $overrideExistingVars = false): void
     {
         $k = $envKey ?? $this->envKey;
 
diff --git a/Exception/FormatException.php b/Exception/FormatException.php
index 3ac77e5..358fbaa 100644
--- a/Exception/FormatException.php
+++ b/Exception/FormatException.php
@@ -20,7 +20,7 @@ final class FormatException extends \LogicException implements ExceptionInterfac
 {
     private $context;
 
-    public function __construct(string $message, FormatExceptionContext $context, int $code = 0, \Throwable $previous = null)
+    public function __construct(string $message, FormatExceptionContext $context, int $code = 0, ?\Throwable $previous = null)
     {
         $this->context = $context;
 
diff --git a/Exception/PathException.php b/Exception/PathException.php
index 4a4d717..e432b2e 100644
--- a/Exception/PathException.php
+++ b/Exception/PathException.php
@@ -18,7 +18,7 @@
  */
 final class PathException extends \RuntimeException implements ExceptionInterface
 {
-    public function __construct(string $path, int $code = 0, \Throwable $previous = null)
+    public function __construct(string $path, int $code = 0, ?\Throwable $previous = null)
     {
         parent::__construct(sprintf('Unable to read the "%s" environment file.', $path), $code, $previous);
     }