HTTP Response Behavior - Cause Browser To Prompt

I have two very similar pieces of ASP.NET code that send a file in an HTTP Reponse to the client. They should cause the browser to prompt to save the file. The first one works, the second one doesn't. The HTTP responses as seen in Fiddler are below.

Working:
HTTP/1.1 200 OK
Cache-Control: private
Content-Length: 228108
Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
X-AspNet-Version: 4.0.30319
content-disposition: attachment; filename=Report.xlsx
Date: Wed, 05 Jan 2011 12:17:48 GMT
<binary data>

Not working:
HTTP/1.1 200 OK
Server: ASP.NET Development Server/10.0.0.0
Date: Wed, 05 Jan 2011 12:19:21 GMT
X-AspNet-Version: 4.0.30319
Content-Length: 228080
content-disposition: attachment; filename=report 2.xlsx
Cache-Control: private
Content-Type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
Connection: Close
<binary data>

When the first one is seen in Fiddler the browser correctly prompts to save the file. When the second one is seen in Fiddler, nothing observable happens in the browser. Same behaviour in both chrome and firefox.

EDIT: ASP.NET code that produces the second response

Response.Buffer = false;
Response.ContentType = @"application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
Response.AppendHeader("content-length", genstream.Length.ToString());
Response.AppendHeader("Content-Disposition", string.Format("attachment; filename={0}.xlsx", filename));
byte[] buffer = new byte[1024];
genstream.Position = 0;
int n;
while ((n = genstream.Read(buffer, 0, 1024) ) > 0)
{
Response.OutputStream.Write(buffer, 0, n);
}