The request is not supported after FolderPicker PickSingleFolderAsync()

I'm trying to open the FolderPicker on a Runtime project for Windows Phone 8.1, but when I call

await folderPicker.PickSingleFolderAsync();

I get this:

The request is not supported. (Exception from HRESULT: 0x80070032)

This is the full method:

public async void openfolder()
        {
            FolderPicker folderPicker = new FolderPicker();
            folderPicker.SuggestedStartLocation = PickerLocationId.Desktop;
            folderPicker.ViewMode = PickerViewMode.List;
            folderPicker.FileTypeFilter.Add(".txt");
            StorageFolder folder = await folderPicker.PickSingleFolderAsync();
        }

The method get fired only one time. Please help, thanks and sorry for bad english.

I resolved my problem: on Windows Phone 8.1

FolderPicker folderPicker = new FolderPicker();
folderPicker.SuggestedStartLocation = PickerLocationId.Desktop;
folderPicker.ViewMode = PickerViewMode.List;
folderPicker.FileTypeFilter.Add(".txt");
folderPicker.PickFolderAndContinue();

I have to use PickFolderAndContinue() not PickSingleFolderAsync()

Now everything functions correctly.