diff --git a/gw_spaceheat/actors/scada.py b/gw_spaceheat/actors/scada.py
index 29b87f6f..bcd620c1 100644
--- a/gw_spaceheat/actors/scada.py
+++ b/gw_spaceheat/actors/scada.py
@@ -853,8 +853,8 @@ def single_machine_state_received(self, payload: SingleMachineState) -> None:
             self._data.recent_machine_states[payload.MachineHandle] =  MachineStates(
                 MachineHandle=payload.MachineHandle,
                 StateEnum=payload.StateEnum,
-                UnixMsList=prev.UnixMsList.append(payload.UnixMs),
-                StateList=prev.StateList.append(payload.State)
+                UnixMsList=prev.UnixMsList + [payload.UnixMs],
+                StateList=prev.StateList + [payload.State]
             )
         else:
             self._data.recent_machine_states[payload.MachineHandle] = MachineStates(