diff --git a/libraries/AC_AttitudeControl/AC_PosControl.cpp b/libraries/AC_AttitudeControl/AC_PosControl.cpp
index 509fb3cf63..94d2528674 100644
--- a/libraries/AC_AttitudeControl/AC_PosControl.cpp
+++ b/libraries/AC_AttitudeControl/AC_PosControl.cpp
@@ -1187,7 +1187,9 @@ float AC_PosControl::crosstrack_error() const
         // crosstrack is the horizontal distance to the closest point to the current track
         const Vector2f vel_unit = _vel_desired.xy().normalized();
         const float dot_error = pos_error * vel_unit;
-        return safe_sqrt(pos_error.length_squared() - sq(dot_error));
+
+        // todo: remove MAX of zero when safe_sqrt fixed
+        return safe_sqrt(MAX(pos_error.length_squared() - sq(dot_error), 0.0));
     }
 }