Cannot serve file with send_from_directory

[nikos]

# Prepare selected file for download...
filepath = '/home/nikos/wsgi/static/files/'
send_from_directory( filepath, filename, as_attachment=True )

'filename' is a valid filename value
i get the following error:

Bad Request
The browser (or proxy) sent a request that this server could not understand.

Why instead of serving the file the error mentions proxy or bad request?
The script is a mod_wsgi script.