下载WebApi时出现ASYNC错误

我有以下代码，用于下载多个文件，创建一个zip文件，并将该文件返回给用户:

//In a WebAPI GET Handler
public async Task<HttpResponseMessage> Get(string id)
{
    try
    {
        var urlList = CacheDictionary<String, List<String>>.Instance[id];
        var helper = new Helper();
        var zipFile = await helper.CreateZipFormUrls(urlList);
        var response = new HttpResponseMessage(HttpStatusCode.OK);
        var stream = new MemoryStream();
        zipFile.Save(stream);
        response.Content = new ByteArrayContent(stream.ToArray());
        response.Content.Headers.ContentDisposition = new ContentDispositionHeaderValue("attachment");
        response.Content.Headers.ContentType = new MediaTypeHeaderValue("application/zip");
        response.Content.Headers.ContentLength = stream.Length;
        response.Content.Headers.ContentDisposition.FileName = "download.zip";
        return response;
    }
    catch (Exception)
    {
        return Request.CreateResponse(HttpStatusCode.BadRequest);
    }
}
//In a Helper Class
public async Task<ZipFile> CreateZipFromUrls(List<string> urlList)
{
    using (var zip = new ZipFile())
    {
        var files = await ReturnFileData(urlList);
        foreach (var file in files)
        {
            var e = zip.AddEntry(GetFileNameFromUrlString(file.Key), file.Value);
        }
        return zip;
    }
}
static Task<Dictionary<string, byte[]>> ReturnFileData(IEnumerable<string> urls)
{
    Dictionary<Uri, Task<byte[]>> dictionary;
    using (var client = new WebClient())
    {
        dictionary = urls.Select(url => new Uri(url)).ToDictionary(
            uri => uri, uri => client.DownloadDataTaskAsync(uri));
        await Task.WhenAll(dictionary.Values);
    }
    return dictionary.ToDictionary(pair => Path.GetFileName(pair.Key.ToString()),
         pair => pair.Value.Result);
}
private string GetFileNameFromUrlString(string url)
{
    var uri = new Uri(url);
    return System.IO.Path.GetFileName(uri.LocalPath);
}

我总是得到:

异步模块或处理程序在异步操作尚未完成时完成

并且在调用下载方法后无法到达任何断点。我做错了什么?我该往哪里看?