PInvoke.GetRawInputData returns -1

uint output = PInvoke.GetRawInputData(
    (Windows.Win32.UI.Input.HRAWINPUT)m.LParam,
    Windows.Win32.UI.Input.RAW_INPUT_DATA_COMMAND_FLAGS.RID_INPUT,
    null,
    ref dwSize,
    (uint)sizeof(Windows.Win32.UI.Input.HRAWINPUT));
Debug.WriteLine("output " +  (int)output);