C# - How To Get Response.ContentType / "text/csv" Causes XML Error

I've got an odd issue.I'm creating a DataTable type my application then converting it to a CSV so the user can download the data.

I set the content type like this:

context.Response.Clear();
context.Response.ContentType = "text/csv";
context.Response.AddHeader("Content-Disposition", "attachment; filename=" + fileName + ".csv");
var responseBuilder = new StringBuilder(data.Rows.Count * data.Columns.Count * 30);

Now, this works well for all data sizes in my development environment, and for small amounts of data on my production environment, but when it gets to a larger size (say, more than about 4000 rows) on production Firefox gives me the error:

XML Parsing Error: no element found