diff --git a/src/AfipCmsSigner.php b/src/AfipCmsSigner.php
index 87af523..f00e3c3 100644
--- a/src/AfipCmsSigner.php
+++ b/src/AfipCmsSigner.php
@@ -15,7 +15,7 @@ class AfipCmsSigner
 
     private ?string $private_key_passphrase;
 
-    public function __construct(string $certificate, string $private_key, string $private_key_passphrase = null)
+    public function __construct(string $certificate, string $private_key, ?string $private_key_passphrase = null)
     {
         $this->certificate = $certificate;
         $this->private_key = $private_key;
diff --git a/src/AfipConfiguration.php b/src/AfipConfiguration.php
index 78ea230..a21b2fd 100644
--- a/src/AfipConfiguration.php
+++ b/src/AfipConfiguration.php
@@ -16,7 +16,7 @@ class AfipConfiguration
 
     private AfipCmsSigner $signer;
 
-    public function __construct(string $cuit = null, string $certificate = null, string $private_key = null, string $private_key_passphrase = null, bool $production_mode = true)
+    public function __construct(?string $cuit = null, ?string $certificate = null, ?string $private_key = null, ?string $private_key_passphrase = null, bool $production_mode = true)
     {
         $this->cuit = $cuit ?? config('afip.cuit');
         $this->certificate = $certificate ?? config('afip.certificate');
diff --git a/src/LaravelAfip.php b/src/LaravelAfip.php
index d32dea1..ef607ba 100755
--- a/src/LaravelAfip.php
+++ b/src/LaravelAfip.php
@@ -8,17 +8,17 @@
 
 class LaravelAfip
 {
-    public function auth(AfipConfiguration $configuration = null): AuthenticationWebService
+    public function auth(?AfipConfiguration $configuration = null): AuthenticationWebService
     {
         return new AuthenticationWebService($configuration);
     }
 
-    public function billing(AfipConfiguration $configuration = null): ElectronicBillingWebService
+    public function billing(?AfipConfiguration $configuration = null): ElectronicBillingWebService
     {
         return new ElectronicBillingWebService($configuration);
     }
 
-    public function padron(AfipConfiguration $configuration = null): PadronWebService
+    public function padron(?AfipConfiguration $configuration = null): PadronWebService
     {
         return new PadronWebService($configuration);
     }
diff --git a/src/WebServices/AuthenticationWebService.php b/src/WebServices/AuthenticationWebService.php
index d394d70..af69067 100644
--- a/src/WebServices/AuthenticationWebService.php
+++ b/src/WebServices/AuthenticationWebService.php
@@ -12,7 +12,7 @@ class AuthenticationWebService
 
     private AfipClient $client;
 
-    public function __construct(AfipConfiguration $configuration = null)
+    public function __construct(?AfipConfiguration $configuration = null)
     {
         $this->configuration = $configuration ?? new AfipConfiguration();
 
diff --git a/src/WebServices/ElectronicBillingWebService.php b/src/WebServices/ElectronicBillingWebService.php
index d8023ed..4a05515 100644
--- a/src/WebServices/ElectronicBillingWebService.php
+++ b/src/WebServices/ElectronicBillingWebService.php
@@ -18,7 +18,7 @@ class ElectronicBillingWebService
 
     private AfipClient $client;
 
-    public function __construct(AfipConfiguration $configuration = null)
+    public function __construct(?AfipConfiguration $configuration = null)
     {
         $this->configuration = $configuration ?? new AfipConfiguration();
 
diff --git a/src/WebServices/PadronWebService.php b/src/WebServices/PadronWebService.php
index d75f2c3..374f43a 100644
--- a/src/WebServices/PadronWebService.php
+++ b/src/WebServices/PadronWebService.php
@@ -23,7 +23,7 @@ class PadronWebService
 
     private AfipClient $padron13Client;
 
-    public function __construct(AfipConfiguration $configuration = null)
+    public function __construct(?AfipConfiguration $configuration = null)
     {
         $this->configuration = $configuration ?? new AfipConfiguration();