diff --git a/src/Model/Schema/ObjectSchema.php b/src/Model/Schema/ObjectSchema.php
index 7f50e14..e323b0e 100644
--- a/src/Model/Schema/ObjectSchema.php
+++ b/src/Model/Schema/ObjectSchema.php
@@ -13,7 +13,7 @@
     public function __construct(
         public array $properties = [],
         public array $required = [],
-        public bool|Schema $additionalProperties = true,
+        public bool|Schema|null $additionalProperties = null,
         public ?int $minProperties = null,
         public ?int $maxProperties = null,
         ?string $title = null,
diff --git a/src/Parser/Schema/Reader.php b/src/Parser/Schema/Reader.php
index a3b34c2..c10014f 100644
--- a/src/Parser/Schema/Reader.php
+++ b/src/Parser/Schema/Reader.php
@@ -230,8 +230,8 @@ private function object(array $data, string $location, array $common): ObjectSch
         $required = isset($data['required'])
             ? array_map(strval(...), Value::list($data['required'], "{$location}/required"))
             : [];
-        $additional = $data['additionalProperties'] ?? true;
-        if (! is_bool($additional)) {
+        $additional = $data['additionalProperties'] ?? null;
+        if ($additional !== null && ! is_bool($additional)) {
             $additional = $this->read($additional, "{$location}/additionalProperties");
         }
 
diff --git a/tests/Schema/ReaderTest.php b/tests/Schema/ReaderTest.php
index 66580dc..5d206ef 100644
--- a/tests/Schema/ReaderTest.php
+++ b/tests/Schema/ReaderTest.php
@@ -137,6 +137,14 @@ public function test_additional_properties_reads_as_boolean_or_schema(): void
     {
         $reader = $this->reader(Version::V3_0);
 
+        $unspecified = $reader->read(['type' => 'object'], '#/x');
+        self::assertInstanceOf(ObjectSchema::class, $unspecified);
+        self::assertNull($unspecified->additionalProperties);
+
+        $open = $reader->read(['type' => 'object', 'additionalProperties' => true], '#/x');
+        self::assertInstanceOf(ObjectSchema::class, $open);
+        self::assertTrue($open->additionalProperties);
+
         $closed = $reader->read(['type' => 'object', 'additionalProperties' => false], '#/x');
         self::assertInstanceOf(ObjectSchema::class, $closed);
         self::assertFalse($closed->additionalProperties);