diff --git a/msg/manual_control_setpoint.msg b/msg/manual_control_setpoint.msg
index 10c66a9227..febe5edc4e 100644
--- a/msg/manual_control_setpoint.msg
+++ b/msg/manual_control_setpoint.msg
@@ -4,14 +4,14 @@ uint8 SWITCH_POS_NONE = 0		# switch is not mapped
 uint8 SWITCH_POS_ON = 1		# switch activated (value = 1)
 uint8 SWITCH_POS_MIDDLE = 2		# middle position (value = 0)
 uint8 SWITCH_POS_OFF = 3		# switch not activated (value = -1)
-int8 MODE_SLOT_NONE = -1		# no mode slot assigned
-int8 MODE_SLOT_1 = 0			# mode slot 1 selected
-int8 MODE_SLOT_2 = 1			# mode slot 2 selected
-int8 MODE_SLOT_3 = 2			# mode slot 3 selected
-int8 MODE_SLOT_4 = 3			# mode slot 4 selected
-int8 MODE_SLOT_5 = 4			# mode slot 5 selected
-int8 MODE_SLOT_6 = 5			# mode slot 6 selected
-int8 MODE_SLOT_MAX = 6			# number of slots plus one
+uint8 MODE_SLOT_NONE = 0		# no mode slot assigned
+uint8 MODE_SLOT_1 = 1			# mode slot 1 selected
+uint8 MODE_SLOT_2 = 2			# mode slot 2 selected
+uint8 MODE_SLOT_3 = 3			# mode slot 3 selected
+uint8 MODE_SLOT_4 = 4			# mode slot 4 selected
+uint8 MODE_SLOT_5 = 5			# mode slot 5 selected
+uint8 MODE_SLOT_6 = 6			# mode slot 6 selected
+uint8 MODE_SLOT_NUM = 6			# number of slots
 uint8 SOURCE_RC = 1			# radio control
 uint8 SOURCE_MAVLINK_0 = 2		# mavlink instance 0
 uint8 SOURCE_MAVLINK_1 = 3		# mavlink instance 1
@@ -61,7 +61,7 @@ uint8 kill_switch		 # throttle kill: _NORMAL_, KILL
 uint8 arm_switch		 # arm/disarm switch: _DISARMED_, ARMED
 uint8 transition_switch	 # VTOL transition switch: _HOVER, FORWARD_FLIGHT
 uint8 gear_switch	 # landing gear switch: _DOWN_, UP
-int8 mode_slot			 # the slot a specific model selector is in
+uint8 mode_slot			 # the slot a specific model selector is in
 uint8 data_source		 # where this input is coming from
 uint8 stab_switch 		 # stabilize switch (only relevant for fixed wings, optional): _MANUAL, STABILIZED
 uint8 man_switch		 # manual switch (only relevant for fixed wings, optional): _STABILIZED_, MANUAL
diff --git a/src/modules/commander/Commander.cpp b/src/modules/commander/Commander.cpp
index 9ae8fe253f..0349882b1a 100644
--- a/src/modules/commander/Commander.cpp
+++ b/src/modules/commander/Commander.cpp
@@ -139,7 +139,7 @@ static float min_stick_change = 0.25f;
 static struct vehicle_status_s status = {};
 static struct actuator_armed_s armed = {};
 static struct safety_s safety = {};
-static int32_t _flight_mode_slots[manual_control_setpoint_s::MODE_SLOT_MAX];
+static int32_t _flight_mode_slots[manual_control_setpoint_s::MODE_SLOT_NUM];
 static struct commander_state_s internal_state = {};
 
 static uint8_t main_state_before_rtl = commander_state_s::MAIN_STATE_MAX;
@@ -2696,12 +2696,12 @@ Commander::set_main_state_rc(const vehicle_status_s &status_local, bool *changed
 	/* we know something has changed - check if we are in mode slot operation */
 	if (sp_man.mode_slot != manual_control_setpoint_s::MODE_SLOT_NONE) {
 
-		if (sp_man.mode_slot >= (int)(sizeof(_flight_mode_slots) / sizeof(_flight_mode_slots[0]))) {
+		if (sp_man.mode_slot > manual_control_setpoint_s::MODE_SLOT_NUM) {
 			warnx("m slot overflow");
 			return TRANSITION_DENIED;
 		}
 
-		int new_mode = _flight_mode_slots[sp_man.mode_slot];
+		int new_mode = _flight_mode_slots[sp_man.mode_slot - 1];
 
 		if (new_mode < 0) {
 			/* slot is unused */
diff --git a/src/modules/sensors/rc_update.cpp b/src/modules/sensors/rc_update.cpp
index 8d129c0853..9df56ac5af 100644
--- a/src/modules/sensors/rc_update.cpp
+++ b/src/modules/sensors/rc_update.cpp
@@ -350,9 +350,8 @@ RCUpdate::rc_poll(const ParameterHandles &parameter_handles)
 			manual.z = math::constrain(_filter_throttle.apply(manual.z), 0.f, 1.f);
 
 			if (_parameters.rc_map_flightmode > 0) {
-
-				/* the number of valid slots equals the index of the max marker minus one */
-				const int num_slots = manual_control_setpoint_s::MODE_SLOT_MAX;
+				/* number of valid slots */
+				const int num_slots = manual_control_setpoint_s::MODE_SLOT_NUM;
 
 				/* the half width of the range of a slot is the total range
 				 * divided by the number of slots, again divided by two
@@ -369,10 +368,10 @@ RCUpdate::rc_poll(const ParameterHandles &parameter_handles)
 				 * will take us to the correct final index.
 				 */
 				manual.mode_slot = (((((_rc.channels[_parameters.rc_map_flightmode - 1] - slot_min) * num_slots) + slot_width_half) /
-						     (slot_max - slot_min)) + (1.0f / num_slots));
+						     (slot_max - slot_min)) + (1.0f / num_slots)) + 1;
 
-				if (manual.mode_slot >= num_slots) {
-					manual.mode_slot = num_slots - 1;
+				if (manual.mode_slot > num_slots) {
+					manual.mode_slot = num_slots;
 				}
 			}