failed to install bootloader on external LVM/LUKS disk

[Stu Card]

Attempting to install 14.04.4 bootloader on sdb (where sdb1 is LVM/LUKS and occupies entire disk after MBR etc.) failed.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubiquity 2.18.8.12
ProcVersionSignature: Ubuntu 4.2.0-27.32~14.04.1-generic 4.2.8-ckt1
Uname: Linux 4.2.0-27-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
CasperVersion: 1.340.2
Date: Sun Apr 3 23:02:56 2016
InstallCmdLine: file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd.lz quiet splash -- maybe-ubiquity
LiveMediaBuild: Ubuntu 14.04.4 LTS "Trusty Tahr" - Release amd64 (20160217.1)
ProcEnviron:
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)