diff --git a/setup.py b/setup.py
index bb91012..d9fb606 100644
--- a/setup.py
+++ b/setup.py
@@ -46,9 +46,9 @@
     license="MIT",
     long_description=long_description,
     url="http://github.com/loyada/typedpy",
-    download_url="https://github.com/loyada/typedpy/archive/v2.27.3.tar.gz",
+    download_url="https://github.com/loyada/typedpy/archive/v2.27.4.tar.gz",
     keywords=["testing", "type-safe", "strict", "schema", "validation"],
-    version="2.27.3",
+    version="2.27.4",
 )
 
 # coverage run --source=typedpy/ setup.py test
diff --git a/tests/test_fast_serialization_2.py b/tests/test_fast_serialization_2.py
index ec07b08..c7aa289 100644
--- a/tests/test_fast_serialization_2.py
+++ b/tests/test_fast_serialization_2.py
@@ -646,8 +646,8 @@ class Foo(Structure, FastSerializable):
 
 def test_additional_serialization(no_defensive_copy_on_get):
     class Foo(Structure, FastSerializable):
-        first_name: String
-        last_name: String
+        first_name: str
+        last_name: str
         age_years: PositiveInt
 
         def _additional_serialization(self) -> dict:
diff --git a/typedpy/serialization/fast_serialization.py b/typedpy/serialization/fast_serialization.py
index 1f175a1..0280572 100644
--- a/typedpy/serialization/fast_serialization.py
+++ b/typedpy/serialization/fast_serialization.py
@@ -120,12 +120,15 @@ def serializer(self):
             if with_undefined
             else res
         )
-        additional = self._additional_serialization() if has_additional_properties else {}
-        return (
+        res = (
             filtered_res
             if serialize_none
             else {k: v for (k, v) in filtered_res.items() if v is not None}
-        ) | additional
+        )
+        if has_additional_properties:
+            res.update(self._additional_serialization())
+
+        return res
 
     cls.serialize = serializer