diff --git a/monasca_log_api/app/base/model.py b/monasca_log_api/app/base/model.py
index 2e618ef8..4a24739e 100644
--- a/monasca_log_api/app/base/model.py
+++ b/monasca_log_api/app/base/model.py
@@ -28,8 +28,8 @@ def serialize_envelope(envelope):
     json = rest_utils.as_json(envelope, ensure_ascii=False)
 
     if six.PY2:
-        raw = unicode(json.replace(r'\\', r'\\\\'), encoding='utf-8',
-                      errors='replace')
+        raw = six.text_type(json.replace(r'\\', r'\\\\'), encoding='utf-8',
+                            errors='replace')
     else:
         raw = json