QFileSystemModel doesn't give list to QCompleter after file system separator is typed

#include <QtWidgets>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QLineEdit *lineEdit = new QLineEdit;
    QCompleter *completer = new QCompleter(lineEdit);
    QFileSystemModel *fileSystemModel = new QFileSystemModel(completer);
    fileSystemModel->setRootPath("");
    completer->setModel(fileSystemModel);
    completer->setCompletionMode(QCompleter::PopupCompletion);
    completer->setMaxVisibleItems(10);
    completer->setWrapAround(true);
    lineEdit->setCompleter(completer);
    lineEdit->show();
    return a.exec();
}

    completer->setModel(new QDirModel(completer));