diff --git a/src/JSONSchemaGenerator/Generator.php b/src/JSONSchemaGenerator/Generator.php
index 309b22b..708494a 100644
--- a/src/JSONSchemaGenerator/Generator.php
+++ b/src/JSONSchemaGenerator/Generator.php
@@ -26,7 +26,7 @@ abstract class Generator
    * @return string
    * @throws \JSONSchemaGenerator\Parsers\Exceptions\UnmappableException
    */
-  public static function from($object, array $config = null): string
+  public static function from($object, ?array $config = null): string
   {
     return (new BaseParser($config))->parse($object)->json();
   }
@@ -38,7 +38,7 @@ public static function from($object, array $config = null): string
    * @return string
    * @throws \JSONSchemaGenerator\Parsers\Exceptions\UnmappableException
    */
-  public static function fromJson(string $jsonString, array $config = null): string
+  public static function fromJson(string $jsonString, ?array $config = null): string
   {
     return (new BaseParser($config))->parse(json_decode($jsonString))->json();
   }
diff --git a/src/JSONSchemaGenerator/Parsers/BaseParser.php b/src/JSONSchemaGenerator/Parsers/BaseParser.php
index 87a1601..a88236c 100644
--- a/src/JSONSchemaGenerator/Parsers/BaseParser.php
+++ b/src/JSONSchemaGenerator/Parsers/BaseParser.php
@@ -42,7 +42,7 @@ class BaseParser
   /**
    * @param array|null $config
    */
-  public function __construct(array $config = null)
+  public function __construct(?array $config = null)
   {
     $this->config = array_merge([
       'schema_id' => null,