QML reads back -1 from QMediaFormat gadget

import QtQuick
import QtMultimedia
Item {​​​​​    
    MediaEncoder {​​​​​  
        id: encoder        
        Component.onCompleted: {​
            console.log(encoder.mediaFormat.audioCodec)
            encoder.mediaFormat.audioCodec = 7
            console.log(encoder.mediaFormat.audioCodec)
        }
    }
}​​​​​