diff --git a/ess/lib_tw.py b/ess/lib_tw.py
index fe9f4d276b276350c86c83b6a9afb0a8ff447040..7d280d6a9fc7bfbce86623fcd3563eb76cae71db 100644
--- a/ess/lib_tw.py
+++ b/ess/lib_tw.py
@@ -46,7 +46,7 @@ class LATTICE:
         '''
 
         # In case file_name_fmap is str
-        if isinstance(file_name_fmap,basestring): file_name_fmap=[file_name_fmap]
+        if isinstance(file_name_fmap,str): file_name_fmap=[file_name_fmap]
 
         # Elem/comm class dict
         dic_cls={'DRIFT'               : DRIFT               ,