Upgrading to Fedora 20 using fedup - what is the ISO format wanted?

I have an iso image of the Fedora 20 install DVD, but when I try to use it I get this:

fedup -v --iso Fedora-20-x86_64-DVD.iso --network 20 usage: fedup <SOURCE> [options] fedup: error: argument --iso: invalid isofile value: 'Fedora-20-x86_64-DVD.iso'

I wrote this exact iso file to a stick. mounted it, and used the "--device DEV" option, and successfully ran the upgrade, so I would like to know for the future what does the "--iso" option really want?