Facebook API调用在iOS上崩溃-Unity3D

我正在学习本教程https://developers.facebook.com/docs/games/unity/unity-tutorial无法弄清楚为什么这会在iOS上崩溃。在编辑器中播放和未在android上测试时没有错误。它在这行FB.API(url,Facebook.HttpMethod.GET,result =>上崩溃，这行完全按照教程中所说的进行复制。

void FBOnLoggedIn(){             
        ParseFacebookUtils.LogInAsync(FB.UserId, FB.AccessToken, FB.AccessTokenExpiresAt);
        currentUser = ParseUser.CurrentUser;
        // Reqest player info and profile picture                                                                           
        FB.API("/me?fields=id,first_name,friends.limit(500).fields(id)", Facebook.HttpMethod.GET, APICallback);  
        LoadPictureAPI(Util.GetPictureURL("me", 128, 128), MyPictureCallback);
        currentUser["FacebookID"] = FB.UserId;
    }
void LoadPictureAPI (string url, LoadPictureCallback callback)
    {
        print ("load pic");
        FB.API(url,Facebook.HttpMethod.GET,result =>
               {
            if (result.Error != null)
            {
                Util.LogError(result.Error);
                return;
            }
            string imageUrl = Util.DeserializePictureURLString(result.Text);
            StartCoroutine(LoadPictureEnumerator(imageUrl,callback));
        });
    }