QTextFormat setProperty(QTextFormat::***Margin, qreal) dosen't convert type.

#include "mainwindow.h"
#include "qtextedit.h"
#include "qtextdocument.h"

#include <QTextFrame>
#include <QTextFrameFormat>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{

    textedit = new QTextEdit;
    setCentralWidget(textedit);

    QTextDocument *doc = textedit->document();
    QTextFrame *rootFrame = doc->rootFrame();
    QTextFrameFormat rootFrameFormat = rootFrame->frameFormat();
    /* 50.0, 30.0 have no problem*/
    rootFrameFormat.setProperty(QTextFrameFormat::FrameLeftMargin, 50);
    rootFrameFormat.setProperty(QTextFrameFormat::FrameTopMargin, 30);
    rootFrame->setFrameFormat(rootFrameFormat);

    /* 50.0, 50 , both of them are okay */
    rootFrameFormat.setLeftMargin(50);
    rootFrame->setFrameFormat(rootFrameFormat);
    }

MainWindow::~MainWindow()
{
}