diff --git a/libraries/AP_Compass/AP_Compass_HIL.cpp b/libraries/AP_Compass/AP_Compass_HIL.cpp
index a6945bf93a..c352d0c23f 100644
--- a/libraries/AP_Compass/AP_Compass_HIL.cpp
+++ b/libraries/AP_Compass/AP_Compass_HIL.cpp
@@ -51,7 +51,7 @@ bool
 AP_Compass_HIL::init(void)
 {
     // register two compass instances
-    for (uint8_t i=0; i<COMPASS_MAX_INSTANCES; i++) {
+    for (uint8_t i=0; i<HIL_NUM_COMPASSES; i++) {
         _compass_instance[i] = register_compass();
     }
     return true;
diff --git a/libraries/AP_Compass/AP_Compass_HIL.h b/libraries/AP_Compass/AP_Compass_HIL.h
index 160fd4b380..0a67b58975 100644
--- a/libraries/AP_Compass/AP_Compass_HIL.h
+++ b/libraries/AP_Compass/AP_Compass_HIL.h
@@ -4,6 +4,12 @@
 
 #include "Compass.h"
 
+#if COMPASS_MAX_INSTANCES == 1
+# define HIL_NUM_COMPASSES 1
+#else
+# define HIL_NUM_COMPASSES 2
+#endif
+
 class AP_Compass_HIL : public AP_Compass_Backend
 {
 public:
@@ -15,7 +21,7 @@ public:
     static AP_Compass_Backend *detect(Compass &compass);
 
 private:
-    uint8_t     _compass_instance[COMPASS_MAX_INSTANCES];
+    uint8_t     _compass_instance[HIL_NUM_COMPASSES];
 };
 
 #endif