diff --git a/pyosys/generator.py b/pyosys/generator.py
index f89819504..c0ccdac5a 100644
--- a/pyosys/generator.py
+++ b/pyosys/generator.py
@@ -168,9 +168,8 @@ pyosys_headers = [
                         "global_negative_id_prefix_storage_",
                         "global_refcount_storage_",
                         "global_free_idx_list_",
-                        "last_created_idx_ptr_",
-                        "last_created_idx_",
                         "builtin_ff_cell_types",
+                        "substrings",
                     }
                 ),
             ),
@@ -455,7 +454,7 @@ class PyosysWrapperGenerator(object):
     ) -> str:
         is_method = isinstance(function, Method)
         function_return_type = function.return_type.format()
-        if class_basename == "Const" and function_return_type in {
+        if class_basename in {"Const","IdString"} and function_return_type in {
             "iterator",
             "const_iterator",
         }: