Export to excel missing css file error in asp.net

Question

Hey..

i m trying to export data from asp.net form to excel using following code

Response.ContentType = "application/vnd.ms-excel";
 Response.AddHeader("content-disposition", "attachment; filename=Report.xls");

It saves the file but while opening it i get error ---Missing css file default.css...

but its a simple asp.net form and i m not using any css file...??

thanx..