diff --git a/aio/aio-proxy/aio_proxy/response/helpers.py b/aio/aio-proxy/aio_proxy/response/helpers.py
index ff435817..1946604b 100755
--- a/aio/aio-proxy/aio_proxy/response/helpers.py
+++ b/aio/aio-proxy/aio_proxy/response/helpers.py
@@ -79,7 +79,9 @@ def evaluate_field(field_value):
 
 
 def string_list_to_string(string_list):
-    if string_list.strip("[]") == "nan" or string_list is None:
+    if string_list is None:
+        return None
+    elif string_list.strip("[]") == "nan":
         return None
     else:
         elements = string_list.strip("[]").split(", ")