await method in task

await method in task

I work on a document downloader in a windows store app, and I have a
problem with tasks. So here is a sample of my code :
Task created and started
...
HttpDownloader httpDownloader = new HttpDownloader(server);
Action<object> action = (object doc ) =>
httpDownloader.DownloadDocument((Document)doc);
Task t1 = new Task(action,selection.doc);
t1.Start();
...
DownloadDocument method
...
FileSavePicker savePicker = new FileSavePicker();
savePicker.SuggestedStartLocation =
PickerLocationId.DocumentsLibrary;
// Dropdown of file types the user can save the file as
savePicker.FileTypeChoices.Add("Application/pdf", new
List<string>() { ".pdf" });
// Default file name if the user does not type one in or select a
file to replace
savePicker.SuggestedFileName = doc.name+"_"+doc.version;
StorageFile file = await savePicker.PickSaveFileAsync(); // Here
an exception is launch.
...
And every time I get :
Element not found (Exception de HRESULT : 0x80070490)
Without task, my code works fine, but since I want to use tasks to manage
the different download, I have this error.