QModelIndex::isValid() returns true for non-existing siblings of an index

QModelIndex::isValid() returns true for non-existing siblings of an index, e.g model.index(0,0).sibling(100,100).isValid()

To reproduce, change this line in of code in spinboxdelegate example:

tableView.setWindowTitle(QObject::tr("Spin Box Delegate")

to:

QString isValidTest = model.index(0,0).sibling(100,100).isValid() ? ":VALID (NOK)" : ":INVALID (OK)";
tableView.setWindowTitle(QObject::tr("Spin Box Delegate") + isValidTest);

Qt4.8.3, msvc2008 -> index is invalid (OK)
Qt5.0.1, msvc2010 -> index is valid (NOK)