Text alignment in qt with drawText() leads to error with fnt-font

Question

I use fnt-font in my qt-application. On every call to drawText() with any alignment I get the following error :

Failed to compute left/right minimum bearings for "" -

It works if I :

• don't use alignment
• use any ttf-font

GCC - C++ - code :

// Load fonts
int fontId = 0;
for(int index = 0; index < static_cast<int>(FontIndex::SIZE); index++)
{
    fontId = QFontDatabase::addApplicationFont(QString("%1%2").arg(s_fontPath).arg(s_fontName[index]));
    if(fontId == -1)
        LOG(ERROR) << "Can't find " << s_fontPath << s_fontName[index] << endl;
}

m_display.m_font.insert(static_cast<int>(0), QFont("System",    9)); // 13
m_display.m_font.insert(static_cast<int>(1), QFont("Terminal", 14)); // 17

m_display.setFont(0);

m_display.drawText(textRect, Qt::AlignCenter, m_text);