diff --git a/api/src/opentrons/protocol_engine/state/module_substates/flex_stacker_substate.py b/api/src/opentrons/protocol_engine/state/module_substates/flex_stacker_substate.py
index 8f9efad4df1..5f655823a95 100644
--- a/api/src/opentrons/protocol_engine/state/module_substates/flex_stacker_substate.py
+++ b/api/src/opentrons/protocol_engine/state/module_substates/flex_stacker_substate.py
@@ -15,4 +15,4 @@ class FlexStackerSubState:
     """
 
     module_id: FlexStackerId
-    labware_name: str
+    labware_name: str = ""
diff --git a/api/src/opentrons/protocol_engine/state/modules.py b/api/src/opentrons/protocol_engine/state/modules.py
index 18bccb24a93..4fe757fbb7a 100644
--- a/api/src/opentrons/protocol_engine/state/modules.py
+++ b/api/src/opentrons/protocol_engine/state/modules.py
@@ -373,6 +373,10 @@ def _add_module_substate(
                 configured_wavelengths=None,
                 reference_wavelength=None,
             )
+        elif ModuleModel.is_flex_stacker(actual_model):
+            self._state.substate_by_module_id[module_id] = FlexStackerSubState(
+                module_id=FlexStackerId(module_id),
+            )
 
     def _update_additional_slots_occupied_by_thermocycler(
         self,