QTextCharFormat fontFamily asserts

#include <QDebug>
#include <QTextDocument>
#include <QTextFrame>

int main(int argc, char** argv)
{

    QTextDocument doc;

    doc.setHtml("Some <b>formatted</b> text.");

    auto frame = doc.rootFrame();

    auto fit = frame->begin();
    auto fend = frame->end();

    while (!fit.atEnd() && fit != fend) {
        auto frame = fit.currentFrame();
        auto block = fit.currentBlock();
        auto bit = block.begin();
        auto bend = block.end();
        while(!bit.atEnd())
        {
            auto f = bit.fragment();
            auto ff = f.charFormat().fontFamily();
            qDebug() << "FRAG" << f.text() << ff;
            ++bit;
        }
        ++fit;
    }
    return 0;
}

            auto ff = f.charFormat().fontFamily();