diff --git a/libraries/AP_Compass/AP_Compass.cpp b/libraries/AP_Compass/AP_Compass.cpp
index 77bada92a3..5cf9e6dc2a 100644
--- a/libraries/AP_Compass/AP_Compass.cpp
+++ b/libraries/AP_Compass/AP_Compass.cpp
@@ -28,7 +28,7 @@
 #include "AP_Compass.h"
 #include "Compass_learn.h"
 
-extern AP_HAL::HAL& hal;
+extern const AP_HAL::HAL& hal;
 
 #ifndef COMPASS_LEARN_DEFAULT
 #define COMPASS_LEARN_DEFAULT Compass::LEARN_NONE