dhclient-script can't find [[: and shopt

[Alvin]

Trying to run dhclient to get an IPv6 address:

sudo dhclient -6 eth0
/sbin/dhclient-script: 55: /sbin/dhclient-script: shopt: not found
/sbin/dhclient-script: 60: /sbin/dhclient-script: [[: not found
/sbin/dhclient-script: 68: /sbin/dhclient-script: shopt: not found

The command gives the above 3 errors, and no lease is accepted.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: isc-dhcp-client 4.3.1-5ubuntu2
ProcVersionSignature: Ubuntu 3.19.0-15.15-generic 3.19.3
Uname: Linux 3.19.0-15-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
CurrentDesktop: KDE
Date: Wed Apr 29 18:25:42 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2013-12-02 (512 days ago)
InstallationMedia: Kubuntu 13.10 "Saucy Salamander" - Release amd64 (20131016.1)
SourcePackage: isc-dhcp
UpgradeStatus: Upgraded to vivid on 2015-04-25 (4 days ago)