diff --git a/libraries/AP_Compass/AP_Compass_Calibration.cpp b/libraries/AP_Compass/AP_Compass_Calibration.cpp
index 75bf834375..aac5a4bf9c 100644
--- a/libraries/AP_Compass/AP_Compass_Calibration.cpp
+++ b/libraries/AP_Compass/AP_Compass_Calibration.cpp
@@ -208,7 +208,7 @@ bool Compass::send_mag_cal_progress(const GCS_MAVLINK& link)
             cal_status == COMPASS_CAL_RUNNING_STEP_ONE ||
             cal_status == COMPASS_CAL_RUNNING_STEP_TWO) {
             uint8_t completion_pct = calibrator.get_completion_percent();
-            auto& completion_mask = calibrator.get_completion_mask();
+            const CompassCalibrator::completion_mask_t& completion_mask = calibrator.get_completion_mask();
             const Vector3f direction;
             uint8_t attempt = _calibrator[compass_id].get_attempt();
 
diff --git a/libraries/AP_Compass/CompassCalibrator.cpp b/libraries/AP_Compass/CompassCalibrator.cpp
index a41ec94946..618639953a 100644
--- a/libraries/AP_Compass/CompassCalibrator.cpp
+++ b/libraries/AP_Compass/CompassCalibrator.cpp
@@ -147,11 +147,6 @@ void CompassCalibrator::update_completion_mask()
     }
 }
 
-CompassCalibrator::completion_mask_t& CompassCalibrator::get_completion_mask()
-{
-    return _completion_mask;
-}
-
 bool CompassCalibrator::check_for_timeout() {
     uint32_t tnow = AP_HAL::millis();
     if(running() && tnow - _last_sample_ms > 1000) {
diff --git a/libraries/AP_Compass/CompassCalibrator.h b/libraries/AP_Compass/CompassCalibrator.h
index 233ecbed74..d65a2d4a00 100644
--- a/libraries/AP_Compass/CompassCalibrator.h
+++ b/libraries/AP_Compass/CompassCalibrator.h
@@ -50,7 +50,7 @@ public:
     enum Rotation get_original_orientation() const { return _orig_orientation; }
 
     float get_completion_percent() const;
-    completion_mask_t& get_completion_mask();
+    const completion_mask_t& get_completion_mask() const { return _completion_mask; }
     enum compass_cal_status_t get_status() const { return _status; }
     float get_fitness() const { return sqrtf(_fitness); }
     float get_orientation_confidence() const { return _orientation_confidence; }