N9: QSystemNetworkInfo::currentMode returns EthernetMode when the phone is connected via USB

I have a Nokia N9 without SIM nor connected WIFI. My application contains this code:

QSystemNetworkInfo* networkInfo = new QtMobility::QSystemNetworkInfo();
QSystemNetworkInfo::NetworkMode activeMode = networkInfo->currentMode();
if (networkInfo->networkStatus(activeMode) != QSystemNetworkInfo::Connected) {
    ...
} else {
    ...
}

When the phone is connected via USB to my PC, currentMode() returns EthernetMode and networkStatus(currentMode()) returns Connected but in reality the network is not available (the error message is "Network session error.").

I'd expect that currentMode() returns UnknownMode or that networkStatus(currentMode()) returns NoNetworkAvailable.