diff --git a/unoptimized_lr/simple_lr_automaton.py b/unoptimized_lr/simple_lr_automaton.py
index 7207ece11d8f4a5784257976e87393459fa46f0d..9887097935a2341cd32267a2c091dcd95fe30c00 100644
--- a/unoptimized_lr/simple_lr_automaton.py
+++ b/unoptimized_lr/simple_lr_automaton.py
@@ -237,7 +237,7 @@ class LR_automaton_internal(Layout):
             ("parse_stack_bus"      , simple_lr_stack.StackLayout(data_width=parameters.W_state, index_width=parameters.W_stackdepth))
         ]
         if(parameters.generating_tree == True):
-            interfaces.append(("index_stack_bus"))
+            interfaces.append(("index_stack_bus", simple_lr_stack.StackLayout(data_width=parameters.W_parsetree_index, index_width=parameters.W_stackdepth)))
         super().__init__(interfaces)
 
 class LR_automaton_external(Layout):