diff --git a/libraries/AP_NavEKF3/AP_NavEKF3.cpp b/libraries/AP_NavEKF3/AP_NavEKF3.cpp
index 9b4886e81f..5061039962 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3.cpp
+++ b/libraries/AP_NavEKF3/AP_NavEKF3.cpp
@@ -777,7 +777,7 @@ void NavEKF3::UpdateFilter(void)
                 // If the primary core is still healthy,then switching is optional and will only be done if
                 // a core with a significantly lower error score can be found
                 float altErrorScore = core[coreIndex].errorScore();
-                if (altCoreAvailable && (!core[primary].healthy() || altErrorScore < lowestErrorScore)) {
+                if (altCoreAvailable && (!core[newPrimaryIndex].healthy() || altErrorScore < lowestErrorScore)) {
                     newPrimaryIndex = coreIndex;
                     lowestErrorScore = altErrorScore;
                 }