Microsoft jscript error

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ASP.NET

Hi all,
I am clicking btnDownloadWebConnector that was present inside asp:Updatepanel and in event of btnDownloadWebConnector_Click i am calling one function like the following
CodeBehind

protected void btnDownloadWebConnector_Click(object sender, EventArgs e)
{
    string filename = Server.MapPath(@"~/ZSLWebConnect/ZSLWebConnectorSetup.msi");
    FileInfo fileToDownload = new FileInfo(filename);
    if (fileToDownload.Exists)
        {
        Response.ClearContent();
        Response.AddHeader("Content-Disposition", "attachment; filename=" + fileToDownload.Name);
        Response.AddHeader("Content-Length", fileToDownload.Length.ToString());
        Response.ContentType = "application/octet-stream";
        Response.TransmitFile(fileToDownload.FullName.ToString()); 
        Response.End();
        }
   }

After clicking the executing the above code i am getting the javascript error:"Microsoft JScript runtime error: Sys.WebForms.PageRequestManagerParserErrorException: The message received from the server could not be parsed. Common causes for this error are when the response is modified by calls to Response.Write(), response filters, HttpModules, or server trace is enabled.
Details: Error parsing near ''.
Can anyone please help me how to solve the above error?
Thanks in advance,
Bala.