diff --git a/libraries/AP_NavEKF2/AP_NavEKF2.cpp b/libraries/AP_NavEKF2/AP_NavEKF2.cpp
index 6c007a5d00..e42624b4b9 100644
--- a/libraries/AP_NavEKF2/AP_NavEKF2.cpp
+++ b/libraries/AP_NavEKF2/AP_NavEKF2.cpp
@@ -526,14 +526,13 @@ bool NavEKF2::healthy(void) const
 }
 
 // returns the index of the primary core
-// return false if no primary core selected
-bool NavEKF2::getPrimaryCoreIndex(uint8_t index) const
+// return -1 if no primary core selected
+int8_t NavEKF2::getPrimaryCoreIndex(void) const
 {
     if (!core) {
-        return false;
+        return -1;
     }
-    index = primary;
-    return true;
+    return primary;
 }
 
 
diff --git a/libraries/AP_NavEKF2/AP_NavEKF2.h b/libraries/AP_NavEKF2/AP_NavEKF2.h
index 7a9cf7d19c..f38ca1d398 100644
--- a/libraries/AP_NavEKF2/AP_NavEKF2.h
+++ b/libraries/AP_NavEKF2/AP_NavEKF2.h
@@ -58,8 +58,8 @@ public:
     bool healthy(void) const;
 
     // returns the index of the primary core
-    // return false if no primary core selected
-    bool getPrimaryCoreIndex(uint8_t index) const;
+    // return -1 if no primary core selected
+    int8_t getPrimaryCoreIndex(void) const;
 
     // Return the last calculated NED position relative to the reference point (m).
     // If a calculated solution is not available, use the best available data and return false