ReadAsAsync in c# throwing error

Question

I am getting an error on AuthTokenRS value = await response.Content.ReadAsAsync(); The error message is:

'HttpContent' does not contain a definition for 'ReadAsAsync' and no extension method 'ReadAsAsync' and no extension method 'ReadAsAsync' accepting a first argument of type 'HttpContent' could be found(are you missing a using directive or an assembly reference?).

Here is my code snippet:

public async Task<HttpResponse<AuthTokenRS>> AuthorizeAsync(string credentials)
{
    using (HttpClient client = new HttpClient())
    {
        client.BaseAddress = new Uri(this.config.Environment);
        client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic", credentials);
        client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("*/*"));

        var args = new Dictionary<string, string>();
        args.Add("grant_type", "client_credentials");
        var content = new FormUrlEncodedContent(args);
        Logger.DebugFormat("POST {0}\n{1}", AuthorizationEndpoint, await content.ReadAsStringAsync());

        var response = await client.PostAsync(AuthorizationEndpoint, content);
        string requestUri = response.RequestMessage.RequestUri.ToString();
        if (response.IsSuccessStatusCode)
        {
            //AuthTokenRS value = await response.Content.ReadAsAsync<AuthTokenRS>();
            AuthTokenRS value = await response.Content.ReadAsAsync<AuthTokenRS>();

            return HttpResponse<AuthTokenRS>.Success(response.StatusCode, value, requestUri);
        }
        else
        {
            return HttpResponse<AuthTokenRS>.Fail(response.StatusCode, await response.Content.ReadAsStringAsync(), requestUri);
        }
    }
}