Index: src/natools-s_expressions-generic_caches.adb
==================================================================
--- src/natools-s_expressions-generic_caches.adb
+++ src/natools-s_expressions-generic_caches.adb
@@ -187,10 +187,14 @@
    -- Printer Interface --
    -----------------------
 
    overriding procedure Open_List (Output : in out Reference) is
    begin
+      if Output.Exp.Is_Empty then
+         Output.Exp.Replace (Create_Tree'Access);
+      end if;
+
       Output.Exp.Update.Data.Append (List_Node);
    end Open_List;
 
 
    overriding procedure Append_Atom
@@ -204,11 +208,13 @@
    end Append_Atom;
 
 
    overriding procedure Close_List (Output : in out Reference) is
    begin
-      Output.Exp.Update.Data.Close_List;
+      if not Output.Exp.Is_Empty then
+         Output.Exp.Update.Data.Close_List;
+      end if;
    end Close_List;
 
 
 
    -------------------------