diff --git a/openapi_core/unmarshalling/schemas/unmarshallers.py b/openapi_core/unmarshalling/schemas/unmarshallers.py
index 941e28cb..9329dc78 100644
--- a/openapi_core/unmarshalling/schemas/unmarshallers.py
+++ b/openapi_core/unmarshalling/schemas/unmarshallers.py
@@ -292,7 +292,7 @@ def _unmarshal_properties(self, value: Any) -> Any:
         if additional_properties is not False:
             # free-form object
             if additional_properties is True:
-                additional_prop_schema = Spec.from_dict({})
+                additional_prop_schema = Spec.from_dict({"nullable": True})
             # defined schema
             else:
                 additional_prop_schema = self.schema / "additionalProperties"
diff --git a/tests/integration/validation/test_petstore.py b/tests/integration/validation/test_petstore.py
index c52feeb3..6925bbf9 100644
--- a/tests/integration/validation/test_petstore.py
+++ b/tests/integration/validation/test_petstore.py
@@ -752,6 +752,7 @@ def test_post_cats(self, spec, spec_dict):
             "ears": {
                 "healthy": pet_healthy,
             },
+            "extra": None,
         }
         data = json.dumps(data_json)
         headers = {
@@ -799,6 +800,7 @@ def test_post_cats(self, spec, spec_dict):
         assert result.body.address.street == pet_street
         assert result.body.address.city == pet_city
         assert result.body.healthy == pet_healthy
+        assert result.body.extra is None
 
     def test_post_cats_boolean_string(self, spec, spec_dict):
         host_url = "https://staging.gigantic-server.com/v1"