Calling modal dialog in network error handling slot when page not found error exists causes segmentation fault

...
QUrl url("http://www.nokia.com/missingPage"); // Malformed URL (404)
QNetworkRequest req(url);
...
connect(reply, SIGNAL(error(QNetworkReply::NetworkError)),this,SLOT(onError(QNetworkReply::NetworkError)));
...

void MyWindow::onError(QNetworkReply::NetworkError code) {
    QMessageBox mb;
    mb.addButton(QMessageBox::Ok);
    mb.setText("Error");
    mb.exec();
  }
}