QFSFileEngine::extension crashes on the default parameters

According to the documentation interface QAbstractFileEngine::extension have null default values for the second and the third parameters (http://qt.nokia.com/doc/4.6/qfsfileengine.html#extension):

bool QFSFileEngine::extension ( Extension extension, const ExtensionOption * option = 0, ExtensionReturn * output = 0 )

But the following code crashes:

#include <QtCore>
int main(int argc, char *argv[ ])
{
QString file("file");
QFSFileEngine* qfsfileengine = new QFSFileEngine(file);
qfsfileengine->extension(QAbstractFileEngine::MapExtension);
return 0;
}