diff --git a/ess/lib_tw.py b/ess/lib_tw.py
index a271fcd5e457f8f12d04361d68879ae0758fc0e2..bd3411314e214d3bf5d3039256ba313920fd8362 100644
--- a/ess/lib_tw.py
+++ b/ess/lib_tw.py
@@ -212,7 +212,7 @@ class LATTICE:
                             except IndexError:
                                 pass
                     else:
-                        self.lst[i].apt = self.lst[i - 1].apt
+                        self.lst[i].apt = self.get_elem_idx(self.lst[i].idx_elem - 1).apt
             except AttributeError:
                 pass