sbuild-createchroot does not honour --keyring=""

Passing --keyring="" does not actually prevent signature downloading/checking, despite sbuild-createchroot(8) indicating:
> Set to an empty string to disable signature checking.

Tested on 0.62.6-1ubuntu2, 0.64.1-1ubuntu4

root@porterbox:~# sbuild-createchroot precise /var/lib/schroot/chroots/precise-amd64/ --keyring="" http://mirror.local/ubuntu
mkdir /var/lib/schroot/chroots
mkdir /var/lib/schroot/chroots/precise-amd64/
I: SUITE: precise
I: TARGET: /var/lib/schroot/chroots/precise-amd64
I: MIRROR: http://repo/
I: Running debootstrap --arch=amd64 --variant=buildd --verbose --include=fakeroot,build-essential --components=main --resolve-deps precise /var/lib/schroot/chroots/precise-amd64 http://mirror.local/ubuntu
I: Retrieving Release
I: Retrieving Release.gpg
E: Failed getting release signature file http://mirror.local/dists/precise/Release.gpg
E: Error running debootstrap at /usr/sbin/sbuild-createchroot line 200.