diff --git a/src/Mapper/Tree/Shell.php b/src/Mapper/Tree/Shell.php
index ca87bafe..08b729c3 100644
--- a/src/Mapper/Tree/Shell.php
+++ b/src/Mapper/Tree/Shell.php
@@ -38,7 +38,7 @@ public static function root(Type $type, mixed $value): self
         return (new self($type))->withValue($value);
     }
 
-    public function child(string $name, Type $type, Attributes $attributes = null): self
+    public function child(string $name, Type $type, ?Attributes $attributes = null): self
     {
         $instance = new self($type);
         $instance->name = $name;
diff --git a/src/Type/Types/ClassStringType.php b/src/Type/Types/ClassStringType.php
index e1404e55..d715d646 100644
--- a/src/Type/Types/ClassStringType.php
+++ b/src/Type/Types/ClassStringType.php
@@ -27,7 +27,7 @@ final class ClassStringType implements StringType, CompositeType
 
     private string $signature;
 
-    public function __construct(ObjectType|UnionType $subType = null)
+    public function __construct(ObjectType|UnionType|null $subType = null)
     {
         if ($subType instanceof UnionType) {
             foreach ($subType->types() as $type) {