diff --git a/EKF/control.cpp b/EKF/control.cpp
index 698029e521..3300bef2ae 100644
--- a/EKF/control.cpp
+++ b/EKF/control.cpp
@@ -1393,11 +1393,12 @@ void Ekf::controlMagFusion()
 		}
 
 		// If optical flow is the only aiding source and GPS checks are failing, then assume that we are operating
-		// indoors and the magnetometer is unreliable.
+		// indoors and the magnetometer is unreliable. Becasue the optical flow sensor is body fixed, absolute yaw
+		// wrt North is not required for navigation and it is safer not to use the magnetometer.
 		if (_control_status.flags.opt_flow
 				&& !_control_status.flags.gps
 				&& !_control_status.flags.ev_pos
-				&& ((_last_gps_fail_us - _time_last_imu) < 5E6)) {
+				&& ((_time_last_imu - _last_gps_pass_us) > 5E6)) {
 			_mag_use_inhibit = true;
 		} else {
 			_mag_use_inhibit = false;