Unknown Server Error - AsyncFileUpload

Hi

I'm using the AsyncFileUpload control. It's working, in that the file is uploaded, but its alerting an alert - Unknown Server Error?

I'm using the following code:

<asp:AsyncFileUpload ID="AsyncFileUpload_FileUpload" runat="server" ClientIDMode="Static"
    Width="400px" UploaderStyle="Traditional" CompleteBackColor = "White" UploadingBackColor="White"
    ThrobberID="imgLoader" onuploadedcomplete="AsyncFileUpload_FileUpload_UploadedComplete" />

And

        string sFilename = System.IO.Path.GetFileName(AsyncFileUpload_FileUpload.FileName);
        AsyncFileUpload_FileUpload.SaveAs(Server.MapPath("~/downloads/") + sFilename);

Any thoughts?

Thanks as always