diff --git a/ArduCopter/Parameters.h b/ArduCopter/Parameters.h
index 32f3a28e88..2f78f8604c 100644
--- a/ArduCopter/Parameters.h
+++ b/ArduCopter/Parameters.h
@@ -407,7 +407,7 @@ public:
     AP_Int16        rtl_alt_final;
     AP_Int16        rtl_climb_min;              // rtl minimum climb in cm
     AP_Int32        rtl_loiter_time;
-    AP_Int8         rtl_alt_type;
+    AP_Enum<ModeRTL::RTLAltType> rtl_alt_type;
 #endif
 
     AP_Int8         failsafe_gcs;               // ground station failsafe behavior
diff --git a/ArduCopter/mode.h b/ArduCopter/mode.h
index a6dcda9c85..6ad1deffde 100644
--- a/ArduCopter/mode.h
+++ b/ArduCopter/mode.h
@@ -1338,7 +1338,7 @@ public:
     void restart_without_terrain();
 
     // enum for RTL_ALT_TYPE parameter
-    enum class RTLAltType {
+    enum class RTLAltType : int8_t {
         RTL_ALTTYPE_RELATIVE = 0,
         RTL_ALTTYPE_TERRAIN = 1
     };
diff --git a/ArduCopter/mode_rtl.cpp b/ArduCopter/mode_rtl.cpp
index 4718bd7421..fc08b39b19 100644
--- a/ArduCopter/mode_rtl.cpp
+++ b/ArduCopter/mode_rtl.cpp
@@ -49,10 +49,12 @@ void ModeRTL::restart_without_terrain()
 ModeRTL::RTLAltType ModeRTL::get_alt_type() const
 {
     // sanity check parameter
-    if (g.rtl_alt_type < 0 || g.rtl_alt_type > (int)RTLAltType::RTL_ALTTYPE_TERRAIN) {
-        return RTLAltType::RTL_ALTTYPE_RELATIVE;
+    switch ((ModeRTL::RTLAltType)g.rtl_alt_type) {
+    case RTLAltType::RTL_ALTTYPE_RELATIVE ... RTLAltType::RTL_ALTTYPE_TERRAIN:
+        return g.rtl_alt_type;
     }
-    return (RTLAltType)g.rtl_alt_type.get();
+    // user has an invalid value
+    return RTLAltType::RTL_ALTTYPE_RELATIVE;
 }
 
 // rtl_run - runs the return-to-launch controller