ASP.NET Response.TransmitFile Failing on First Download

I have some code on an aspx page then when users loads the page it starts downloading a zip. Looks like this:

            Response.ContentType = "application/octet-stream";
            Response.AppendHeader("Content-Disposition", "attachment; filename=" + fileSaveName);
            Response.TransmitFile(zipPath);
            Response.End();

The problem is the FIRST time this is hit I get the following error:

The zip file exists and is not in my wwwroot. If I refresh the page the file will download fine.

If I wrap the code in a Try Catch I get a System.Threading.Threadabort exception with the message:

Unable to evaluate expression because the code is optimized or a native frame is on top of the call stack.

N开发者_高级运维OTE: I actually get that exception every time. I guess its just to do with the Response.End

Thanks for any help!

Response.End would probably throw an exception to get out quick'n'dirty so that's fine, however, that file.. I suspect your Response.End is messing things up, have you tried to do a .Flush() first? You might get a clue by using Fiddler to examine what the server actually returns