WWW class does not work (unity)

public class Controller : MonoBehaviour {

    // Use this for initialization
    public string url = "http://ift.tt/1iBlctx";
    IEnumerator Start() {
        WWW www = new WWW(url);

        yield return www;

        Debug.Log (www.bytes.Length);
    }

WWW class does not work, it returns 0 bytes. What's the problem? This code is the example code of www class in unity docs.