QTextBlockFormat: vertical alignment doesn't work

QTextFrameFormat frameFormat;
 frameFormat.setHeight(400);
 frameFormat.setBorder(2.0);
 frameFormat.setBorderBrush(Qt::black);
QTextFrame* frame = cursor.insertFrame(frameFormat);
QTextBlockFormat blockFormat;
 blockFormat.setAlignment(Qt::AlignCenter); // doesn't work
 cursor.setBlockFormat(blockFormat);
QTextCharFormat charFormat;
 charFormat.setFontPointSize(30);
 cursor.insertText("text", charFormat);