ValueError: could not convert string to float: k_latency=0.850

/etc/cron.daily/apt:
Traceback (most recent call last):
  File "/usr/sbin/update-apt-xapian-index", line 101, in <module>
    if not indexer.setupIndexing(force=opts.force, system=opts.pkgfile is None):
  File "/usr/lib/python2.7/dist-packages/axi/indexer.py", line 513, in setupIndexing
    addon.obj.init(dict(values=self.values), self.progress)
  File "/usr/share/apt-xapian-index/plugins/cataloged_time.py", line 72, in init
    self._package_cataloged_time = cPickle.load(open(self._packages_cataloged_file))
ValueError: could not convert string to float: k_latency=0.850

This error is thrown from time to time from all versions of Ubuntu from 10.10 on upward:
Ubuntu 10.10
Ubuntu 11.04
Ubuntu 11.10
Ubuntu 12.04

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: apt 0.8.16~exp5ubuntu13.2
ProcVersionSignature: Ubuntu 3.0.0-17.30-virtual 3.0.22
Uname: Linux 3.0.0-17-virtual x86_64
ApportVersion: 1.23-0ubuntu4
Architecture: amd64
Date: Mon Apr 2 22:08:38 2012
InstallationMedia: Ubuntu-Server 10.04.1 LTS "Lucid Lynx" - Release amd64 (20100816.2)
ProcEnviron:
 PATH=(custom, user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: apt
UpgradeStatus: Upgraded to oneiric on 2011-09-16 (199 days ago)