Microsoft Cognitive Service Vision API ClientException Error

I'm trying to use Vision Cognitive Services to receive the description of an image but my code always throws this exception:

Exception Microsoft.ProjectOxford.Vision.ClientException
HResult=0x80131500
Origine=<Non è possibile valutare l'origine dell'eccezione>
Stack:
in Microsoft.ProjectOxford.Vision.VisionServiceClient.HandleException (Exception exception)
in Microsoft.ProjectOxford.Vision.VisionServiceClient.<SendAsync>b__42_1[TRequest,TResponse](Exception e)
in System.AggregateException.Handle(Func`2 predicate)
in Microsoft.ProjectOxford.Vision.VisionServiceClient.<SendAsync>d__42`2.MoveNext()
in System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
in Microsoft.ProjectOxford.Vision.VisionServiceClient.<AnalyzeImageAsync>d__21`1.MoveNext()
in System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
in Microsoft.ProjectOxford.Vision.VisionServiceClient.<AnalyzeImageAsync>d__20.MoveNext()
in System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
in System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
in CognitiveServices.MainPage.<Button_Clicked>d__1.MoveNext() in C:\Users\manu9\documents\visual studio 2017\Projects\CognitiveServices\CognitiveServices\CognitiveServices\MainPage.xaml.cs: riga 48

This is my code:

using Microsoft.ProjectOxford.Vision;
using Microsoft.ProjectOxford.Vision.Contract;
using Plugin.Media;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Forms;

namespace CognitiveServices
{
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
        }

        private async void Button_Clicked(object sender, EventArgs e)
        {
            var media = Plugin.Media.CrossMedia.Current;
            await media.Initialize();
            var file = await media.TakePhotoAsync(new Plugin.Media.Abstractions.StoreCameraMediaOptions
            {
                SaveToAlbum = false
            });
            image.Source = ImageSource.FromStream(() => file.GetStream());

            var visionClient = new VisionServiceClient("MY_API_KEY");

            var visualFeats = new VisualFeature[]
            {
                VisualFeature.Description, 
                VisualFeature.Faces
            };

            Stream imagestream = file.GetStream();
            imagestream.Seek(0, SeekOrigin.Begin);

            var result = await visionClient.AnalyzeImageAsync(imagestream, visualFeats);
            description.Text = result.Description.Captions.First().Text;

            Debug.WriteLine(result.Description.Captions[0].Text);
            file.Dispose();
        }
    }
}

Why do I have always this Exception? I read that someone resolved this by adding something like imageStream.Seek(0) is it true?