diff --git a/langfuse/_client/attributes.py b/langfuse/_client/attributes.py
index 1c22b7518..0438b959a 100644
--- a/langfuse/_client/attributes.py
+++ b/langfuse/_client/attributes.py
@@ -152,7 +152,10 @@ def create_generation_attributes(
 
 
 def _serialize(obj: Any) -> Optional[str]:
-    return json.dumps(obj, cls=EventSerializer) if obj is not None else None
+    if obj is None or isinstance(obj, str):
+        return obj
+
+    return json.dumps(obj, cls=EventSerializer)
 
 
 def _flatten_and_serialize_metadata(
diff --git a/tests/test_decorators.py b/tests/test_decorators.py
index 6598bac55..fe0a7f4c3 100644
--- a/tests/test_decorators.py
+++ b/tests/test_decorators.py
@@ -905,7 +905,7 @@ async def level_1_function(*args, **kwargs):
     assert generation.usage.output is not None
     assert generation.usage.total is not None
     print(generation)
-    assert generation.output == "2"
+    assert generation.output == 2
 
 
 def test_generator_as_function_input():
diff --git a/tests/test_openai.py b/tests/test_openai.py
index 86b0f057c..85205db28 100644
--- a/tests/test_openai.py
+++ b/tests/test_openai.py
@@ -119,7 +119,7 @@ def test_openai_chat_completion_stream(openai):
     assert generation.data[0].usage.input is not None
     assert generation.data[0].usage.output is not None
     assert generation.data[0].usage.total is not None
-    assert generation.data[0].output == "2"
+    assert generation.data[0].output == 2
     assert generation.data[0].completion_start_time is not None
 
     # Completion start time for time-to-first-token
@@ -179,7 +179,7 @@ def test_openai_chat_completion_stream_with_next_iteration(openai):
     assert generation.data[0].usage.input is not None
     assert generation.data[0].usage.output is not None
     assert generation.data[0].usage.total is not None
-    assert generation.data[0].output == "2"
+    assert generation.data[0].output == 2
     assert generation.data[0].completion_start_time is not None
 
     # Completion start time for time-to-first-token