diff --git a/doc/changelog.rst b/doc/changelog.rst
index 136b503..5914862 100644
--- a/doc/changelog.rst
+++ b/doc/changelog.rst
@@ -1,6 +1,13 @@
 Changelog
 =========
 
+[0.6.5] - 2026-03-10
+--------------------
+
+Fixed
+^^^^^
+- Fix extension serialization crash when an extension is declared but not populated on a resource serialized outside of SCIM context (e.g. FastAPI ``response_model``). :pr:`131`
+
 [0.6.4] - 2026-02-05
 --------------------
 
diff --git a/scim2_models/resources/resource.py b/scim2_models/resources/resource.py
index 254f314..3e50ccb 100644
--- a/scim2_models/resources/resource.py
+++ b/scim2_models/resources/resource.py
@@ -117,7 +117,11 @@ def _extension_serializer(
     For instance, attributes 'meta', 'id' or 'schemas' should not be
     dumped when the model is used as an extension for another model.
     """
+    if value is None:
+        return None
+
     partial_result = handler(value)
+
     result = {
         attr_name: value
         for attr_name, value in partial_result.items()
diff --git a/tests/test_resource_extension.py b/tests/test_resource_extension.py
index 01983f5..a27d998 100644
--- a/tests/test_resource_extension.py
+++ b/tests/test_resource_extension.py
@@ -1,6 +1,7 @@
 import datetime
 
 import pytest
+from pydantic import TypeAdapter
 
 from scim2_models import URN
 from scim2_models import Context
@@ -370,6 +371,14 @@ def test_class_getitem():
         User[int]
 
 
+def test_dump_resource_with_unset_extension():
+    """Serialize a resource whose extension is declared but not populated."""
+    user = User[EnterpriseUser](user_name="bjensen")
+    ta = TypeAdapter(User[EnterpriseUser])
+    payload = ta.dump_python(user)
+    assert "urn:ietf:params:scim:schemas:extension:enterprise:2.0:User" not in payload
+
+
 def test_model_attribute_to_scim_attribute_error():
     """Test error case where get_field_root_type returns None."""
     from pydantic import Field