Issue with QList and type of index use to get value from list

This simple code give it to me strange result.

main.cpp:

#include <QCoreApplication>
#include <QStringList>

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

    unsigned int    unsignedIndex = 1;
    QList<float>    floatList;
    QStringList     variantList;
    int j = 0;

    floatList << 99.4f << 85.6f;
    variantList << QString("First Object") << QString("Second Object") ;

    for(j = 0; j < variantList.count(); j++){
        QString testVar = variantList.at(j);

        if( (0 != j) && (0 != unsignedIndex))
        {
            qDebug("Float val from int index : %.2f", floatList.at((j-1)));
            qDebug("Float val from unsigned int index : %.2f", floatList.at(unsignedIndex - 1));
        }

        qDebug(testVar.toUtf8());

        unsignedIndex++;
    }

    return 0;
}

Project file (.pro):

QT       += core
QT       -= gui

TARGET = TestCompilerIssue
CONFIG   += console

TEMPLATE = app
CONFIG += x86

SOURCES += main.cpp

The result is different when I compile this example in debug and release mode.
In debug mode I get the following correct result in the console:

First Object
Float val from int index : 99.40
Float val from unsigned int index : 85.60
Second Object

In release mode I get the following incorrect result in the console:

First Object
Float val from int index : 0.00
Float val from unsigned int index : 85.60
Second Object

I don't understand why I get the following result because every variables are initialised correctly.
I suppose that it is a compiler issue but I am not sure. As you can see use an int or an unsigned int give different result.

I use the QtCreator 2.0.1 with the default compiler GCC under Mac OS 10.6.X.
I was not able to reproduce this issue under any Windows OS (XP, Vista, Seven).
I was not able to check if this issue is reproductible under other Mac OS like 10.5.

Thank you for your help.