QString::arg: Argument missing

[BalaQT]

hi,
I'm using linguist to translate the following, (environment: ubuntu,qt4.7)

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    QTranslator translator;
    QString txt1="text";
    translator.load("testLang");
    qApp->installTranslator(&translator);
    ui->setupUi(this);
    l=new QLabel(this);
    l->setText(tr("file %1").arg(txt1));
}

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    QTranslator translator;
    QString txt1="text";
    translator.load("testLang");
    qApp->installTranslator(&translator);
    ui->setupUi(this);
    l=new QLabel(this);
    l->setText(tr("file %1").arg(txt1));
}

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But I'm getting the following warning:
QString::arg: Argument missing: test translatedText, text
(translatedText: The text translated in the target language.)
what's the possible cause , how can i get rid of this?
Thanks in advance.
Bala