Saving file on server problem...

Hi,
I'm tring to save file on server,
     FileStream fs = new FileStream(context.Server.MapPath("~/files/" + fileName), chunk == 0 ? FileMode.OpenOrCreate : FileMode.Append);
but I'm getting this error:
System.IO.DirectoryNotFoundException was unhandled by user code  Message=Could not find a part of the path...
The folder is exist on project folder.
What to do?
Thanks..
...