diff --git a/gtsam/navigation/MagFactor.h b/gtsam/navigation/MagFactor.h
index 79677c0c6..c2ccb31ff 100644
--- a/gtsam/navigation/MagFactor.h
+++ b/gtsam/navigation/MagFactor.h
@@ -54,8 +54,11 @@ public:
   static Point3 unrotate(const Rot2& R, const Point3& p,
       boost::optional<Matrix&> HR = boost::none) {
     Point3 q = Rot3::yaw(R.theta()).unrotate(p, HR);
-    if (HR)
-      *HR = HR->col(2);
+    if (HR) {
+      // assign to temporary first to avoid error in Win-Debug mode
+	  Matrix H  = HR->col(2);
+      *HR = H;
+	}
     return q;
   }