QTreeView: expandAll() and expandToDepth(int) not working

[papillon]

I've the following code (QT 4.7):

QFileSystemModel *treeModel = new QFileSystemModel;
        QTreeView *libTree = new QTreeView(mainFrame);
        libTree->setModel(treeModel);
	libTree->setItemsExpandable(true);
	libTree->expandAll();

QFileSystemModel *treeModel = new QFileSystemModel;
        QTreeView *libTree = new QTreeView(mainFrame);
        libTree->setModel(treeModel);
	libTree->setItemsExpandable(true);
	libTree->expandAll();

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expandAll() doesn't produce what I expected (i.e. automatically expand all nodes), and expandToDepth(int) doesn't work either. Any idea why? Thanks