ubiquity crashes on install step

[Tim Towers]

running ubiquity from an ubuntu 16.04.2 installed to a USB stick.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubiquity 2.21.63.3
ProcVersionSignature: Ubuntu 4.8.0-52.55~16.04.1-generic 4.8.17
Uname: Linux 4.8.0-52-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.6
Architecture: amd64
Date: Sat May 27 20:23:26 2017
InstallCmdLine: file=/cdrom/preseed/lubuntu.seed boot=casper only-ubiquity initrd=/casper/initrd.lz quiet splash ---
InstallationDate: Installed on 2017-05-16 (11 days ago)
InstallationMedia: Lubuntu 16.04.2 LTS "Xenial Xerus" - Release amd64 (20170215)
ProcEnviron:
 LANGUAGE=en_GB.UTF-8
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)