Render dot to png generated empty file

[fengttt]

I am trying to render a dot file to png. Here is my simple function.

func RenderDot(dot string, fn string) error {
	ctx := context.Background()
	g, err := graphviz.New(ctx)
	if err != nil {
		return err
	}
	defer g.Close()
	graph, err := graphviz.ParseBytes([]byte(dot))
	if err != nil {
		return err
	}

	err = g.RenderFilename(ctx, graph, graphviz.PNG, fn)
	return err
}

When trying to render "digraph G {Hello->World}", there is no error, but the generated png file is 0 byte. Any hint that what I did wrong?