Problems on drawing text of big font size on QGLWidget and QGLFramebufferObject

#include <QtGui/QApplication>
#include <QtOpenGL/QGLWidget>
#include <QtOpenGL/QGLFramebufferObject>

class GLWidget : public QGLWidget {
public:
	GLWidget() {
		m_big = font();
		m_big.setPixelSize(100);
		makeCurrent();
		m_fbo = new QGLFramebufferObject(500, 150);
		m_fbo->bind();
		glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
		glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
		QPainter painter(m_fbo);
		painter.setFont(m_big);
		painter.drawText(QRect(0, 0, 500, 150), "Test Text");
		m_fbo->release();
	}
	~GLWidget() {
		delete m_fbo;
	}
private:
	void paintEvent(QPaintEvent *) {
		QPainter painter(this);
		painter.setFont(m_big);
		painter.drawText(QRect(0, 0, 500, 150), "Test Text");
		drawTexture(QPointF(0.0, 150.0), m_fbo->texture());
	}
	QFont m_big;
	QGLFramebufferObject *m_fbo;
};

int main(int argc, char *argv[]) {
	QApplication app(argc, argv);
	GLWidget w;
	w.show();
	return app.exec();
}