I've encountered the issue as well, and this change - https://code.launchpad.net/~ubuntu-branches/ubuntu/wily/ubuntu-release-upgrader/wily-proposed/+merge/266658 - solves the problem for me.
I've encountered the issue as well, and this change - https:/ /code.launchpad .net/~ubuntu- branches/ ubuntu/ wily/ubuntu- release- upgrader/ wily-proposed/ +merge/ 266658 - solves the problem for me.