Alpha cache is not cleared when printing to PDF

bool QPdfEngine::begin(QPaintDevice *pdev)
{
    Q_D(QPdfEngine);

    if(!QPdfBaseEngine::begin(pdev)) {
        state = QPrinter::Error;
        return false;
    }
    d->stream->setDevice(d->outDevice);

    d->streampos = 0;
    d->hasPen = true;
    d->hasBrush = false;
    d->clipEnabled = false;
    d->allClipped = false;

    d->xrefPositions.clear();
    d->pageRoot = 0;
    d->catalog = 0;
    d->info = 0;
    d->graphicsState = 0;
    d->patternColorSpace = 0;

    d->pages.clear();
    d->imageCache.clear();
    // missing - should be added:
    d->alphaCache.clear();