TikZ external library doesn't update figures.

After upgrading to 14.04 from 13.10, the TikZ external library has stopped rebuilding pdf figure files (I'm using PDFLaTeX) when the associated <filename>.tikz source file has been changed. Indeed, the <filename>-output.md5 file is not being generated for the figure files when PDFLaTeX is run.

texlive-full version: 2013.20140215-1 0

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: texlive-full 2013.20140215-1
ProcVersionSignature: Ubuntu 3.13.0-24.46-generic 3.13.9
Uname: Linux 3.13.0-24-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.14.1-0ubuntu3
Architecture: amd64
CurrentDesktop: Unity
Date: Tue Apr 29 11:49:50 2014
InstallationDate: Installed on 2011-01-21 (1194 days ago)
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
PackageArchitecture: all
SourcePackage: texlive-base
UpgradeStatus: Upgraded to trusty on 2014-04-18 (10 days ago)