diff --git a/libraries/AP_Mission/AP_Mission.h b/libraries/AP_Mission/AP_Mission.h
index b436093fe8..0379aa324c 100644
--- a/libraries/AP_Mission/AP_Mission.h
+++ b/libraries/AP_Mission/AP_Mission.h
@@ -247,7 +247,7 @@ public:
     /// get_prev_nav_cmd_index - returns the previous "navigation" commands index (i.e. position in the mission command list)
     ///     if there was no previous nav command it returns AP_MISSION_CMD_INDEX_NONE
     ///     we do not return the entire command to save on RAM
-    uint16_t get_prev_nav_cmd_index() { return _prev_nav_cmd_index; }
+    uint16_t get_prev_nav_cmd_index() const { return _prev_nav_cmd_index; }
 
     /// get_next_nav_cmd - gets next "navigation" command found at or after start_index
     ///     returns true if found, false if not found (i.e. reached end of mission command list)