Out parameters of stored procedures return empty string

#include <QCoreApplication>
#include <QtSql>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QSqlDatabase db = QSqlDatabase::addDatabase("QODBC");
    QString connectionStr("Driver={SQL Server Native Client 10.0};Server=.\\SqlExpress;Database=testdb;Uid=username;Pwd=password;");
    db.setDatabaseName(connectionStr);

    if(db.open())
    {
        QString testStr;
        testStr.reserve( 100 );

        QSqlQuery query;
        query.prepare("{CALL getTestString(?)}");
        query.bindValue("testStr", testStr, QSql::Out);
        if( query.exec() )
        {
            testStr = query.boundValue("testStr").toString();
            if( testStr.isEmpty() )
                qDebug() << "String is empty"; //always return empty string for Qt 4.7.0
            else
                qDebug() << testStr;//works fine with version 4.6.1
        }
        else
        {
            qDebug() << query.lastError().text();
        }
    }
    else
    {
        qDebug() << "Can't connect";
    }
    return 0;
}

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE PROCEDURE getTestString 
	@key nvarchar(50) OUTPUT
AS
BEGIN
	SET NOCOUNT ON;
	SET @key = 'TEST'
END
GO