Geoposition.event.heading always return null value on IOS6/AIR34

I'm building an ios app with Flash CS6 / AIR3.4.

(...)

        private function geoUpdated(e:GeolocationEvent):void {

            latitudeField.text = e.latitude.toString();

            longitudeField.text = e.longitude.toString();

            altitudeField.text = e.altitude.toString();

            hAccuracyField.text = e.horizontalAccuracy.toString();

            vAccuracyField.text = e.verticalAccuracy.toString();

            timestampField.text = e.timestamp.toString();

            headingField.text = e.heading.toString();

        }

(../...)

When I use it on my iPodTouch device with IOS6, the value of e.heading is always a null (the other values of geoposition are correct).

I'm using Flash CS6 and AIR 3.4

Could you help me?

Thanks