diff --git a/langfuse/_utils/serializer.py b/langfuse/_utils/serializer.py
index 0d6237f1c..e4b625a6e 100644
--- a/langfuse/_utils/serializer.py
+++ b/langfuse/_utils/serializer.py
@@ -63,7 +63,7 @@ def default(self, obj: Any):
                 return obj.tolist()
 
             if isinstance(obj, float) and math.isnan(obj):
-                return None
+                return "NaN"
 
             if isinstance(obj, float) and math.isinf(obj):
                 return "Infinity"