diff --git a/iso15118/secc/controller/interface.py b/iso15118/secc/controller/interface.py
index 08ab6498..0979602d 100644
--- a/iso15118/secc/controller/interface.py
+++ b/iso15118/secc/controller/interface.py
@@ -160,9 +160,7 @@ async def get_schedule_exchange_params(
         selected_energy_service: SelectedEnergyService,
         control_mode: ControlMode,
         schedule_exchange_req: ScheduleExchangeReq,
-    ) -> Optional[
-        Union[ScheduledScheduleExchangeResParams, DynamicScheduleExchangeResParams]
-    ]:
+    ) -> Union[ScheduledScheduleExchangeResParams, DynamicScheduleExchangeResParams]:
         """
         Gets the parameters for a ScheduleExchangeResponse.
         If the parameters are not yet ready when requested,
diff --git a/iso15118/secc/controller/simulator.py b/iso15118/secc/controller/simulator.py
index 389c052e..f85b62a9 100644
--- a/iso15118/secc/controller/simulator.py
+++ b/iso15118/secc/controller/simulator.py
@@ -298,9 +298,7 @@ async def get_schedule_exchange_params(
         selected_energy_service: SelectedEnergyService,
         control_mode: ControlMode,
         schedule_exchange_req: ScheduleExchangeReq,
-    ) -> Optional[
-        Union[ScheduledScheduleExchangeResParams, DynamicScheduleExchangeResParams]
-    ]:
+    ) -> Union[ScheduledScheduleExchangeResParams, DynamicScheduleExchangeResParams]:
         if control_mode == ControlMode.SCHEDULED:
             return await self.get_scheduled_se_params(
                 selected_energy_service, schedule_exchange_req
@@ -314,7 +312,7 @@ async def get_scheduled_se_params(
         self,
         selected_energy_service: SelectedEnergyService,
         schedule_exchange_req: ScheduleExchangeReq,
-    ) -> Optional[ScheduledScheduleExchangeResParams]:
+    ) -> ScheduledScheduleExchangeResParams:
         """Overrides EVSEControllerInterface.get_scheduled_se_params()."""
         charging_power_schedule_entry = PowerScheduleEntry(
             duration=3600,
@@ -510,7 +508,7 @@ async def get_dynamic_se_params(
         self,
         selected_energy_service: SelectedEnergyService,
         schedule_exchange_req: ScheduleExchangeReq,
-    ) -> Optional[DynamicScheduleExchangeResParams]:
+    ) -> DynamicScheduleExchangeResParams:
         """Overrides EVSEControllerInterface.get_dynamic_se_params()."""
         price_level_schedule_entry = PriceLevelScheduleEntry(
             duration=3600, price_level=1
diff --git a/iso15118/secc/states/iso15118_20_states.py b/iso15118/secc/states/iso15118_20_states.py
index 2bc0207f..1d538422 100644
--- a/iso15118/secc/states/iso15118_20_states.py
+++ b/iso15118/secc/states/iso15118_20_states.py
@@ -53,6 +53,7 @@
     CertificateInstallationReq,
     ChargeProgress,
     ChargingSession,
+    DynamicScheduleExchangeResParams,
     EIMAuthSetupResParams,
     EVPowerProfile,
     MatchedService,
@@ -457,7 +458,6 @@ async def process_message(
         if (
             current_authorization_status.authorization_status
             == AuthorizationStatus.ACCEPTED
-            and self.comm_session.evse_controller.ready_to_charge()
         ):
             evse_processing = Processing.FINISHED
         elif (
@@ -931,25 +931,39 @@ async def process_message(
         ev_data_context.update_schedule_exchange_parameters(
             control_mode, schedule_exchange_req
         )
-        evse_processing = Processing.ONGOING
+
+        # As per Table 49 of ISO15118-20 spec: one of scheduled_params/dynamic_params is
+        # required even if EVSEProcessing is ongoing. The SECC shall only omit the
+        # parameter 'ScheduleList' in case EVSEProcessing is set to 'Ongoing'.
+        # However, the schema file doesn't permit this as minOccurs = 0 is not set in
+        # schema here: https://github.com/SwitchEV/iso15118/blob/769eddb0cb780db629b4c736de270d381516abd1/iso15118/shared/schemas/iso15118_20/V2G_CI_CommonMessages.xsd#L467-L466  # noqa
         params = await self.comm_session.evse_controller.get_schedule_exchange_params(
             self.comm_session.selected_energy_service,
             control_mode,
             schedule_exchange_req,
         )
-        if params:
+
+        if (
+            control_mode == ControlMode.SCHEDULED
+            and type(params) is not ScheduledScheduleExchangeResParams
+        ) or (
+            control_mode == ControlMode.DYNAMIC
+            and type(params) is not DynamicScheduleExchangeResParams
+        ):
+            self.stop_state_machine(
+                f"Unexpected control_mode {control_mode},"
+                f" for params type {type(params)}",
+                message,
+                ResponseCode.FAILED,
+            )
+            return
+
+        if self.comm_session.evse_controller.ready_to_charge():
             evse_processing = Processing.FINISHED
-            if control_mode == ControlMode.SCHEDULED:
-                if type(params) is ScheduledScheduleExchangeResParams:
-                    self.comm_session.offered_schedules_V20 = params.schedule_tuples
-                else:
-                    self.stop_state_machine(
-                        f"Unexpected control_mode {control_mode},"
-                        f" for params type {type(params)}",
-                        message,
-                        ResponseCode.FAILED,
-                    )
-                    return
+            if type(params) is ScheduledScheduleExchangeResParams:
+                self.comm_session.offered_schedules_V20 = params.schedule_tuples
+        else:
+            evse_processing = Processing.ONGOING
 
         schedule_exchange_res = ScheduleExchangeRes(
             header=MessageHeader(
@@ -960,10 +974,12 @@ async def process_message(
             scheduled_params=params if control_mode == ControlMode.SCHEDULED else None,
             dynamic_params=params if control_mode == ControlMode.DYNAMIC else None,
         )
-        evse_data_context = self.comm_session.evse_controller.evse_data_context
-        evse_data_context.update_schedule_exchange_parameters(
-            control_mode, schedule_exchange_res
-        )
+
+        if evse_processing == Processing.FINISHED:
+            evse_data_context = self.comm_session.evse_controller.evse_data_context
+            evse_data_context.update_schedule_exchange_parameters(
+                control_mode, schedule_exchange_res
+            )
 
         # We don't know what request will come next (which state to transition to),
         # unless the schedule parameters are ready and we're in AC charging.