diff --git a/clearml_agent/helper/dicts.py b/clearml_agent/helper/dicts.py
index 4fc4cbd..80b76b7 100644
--- a/clearml_agent/helper/dicts.py
+++ b/clearml_agent/helper/dicts.py
@@ -14,7 +14,7 @@ def merge_dicts(dict1, dict2, custom_merge_func=None):
         return dict2
     for k in dict2:
         if k in dict1:
-            res = None
+            res = _not_set
             if custom_merge_func:
                 res = custom_merge_func(k, dict1[k], dict2[k], _not_set)
             dict1[k] = merge_dicts(dict1[k], dict2[k], custom_merge_func) if res is _not_set else res