Ubiquity crashes for Kubuntu installation on Live ISO

Installing Kubuntu LTS 18.04, the installer consistently crashes with the following stack trace:

Traceback (most recent call last):
  File "/usr/lib/ubiquity/bin/ubiquity", line 655, in <module>
    main(oem_config)
  File "/usr/lib/ubiquity/bin/ubiquity", line 639, in main
    install(args[0], query=options.query)
  File "/usr/lib/ubiquity/bin/ubiquity", line 270, in install
    wizard = ui.Wizard(distro)
  File "/usr/lib/ubiquity/ubiquity/frontend/kde_ui.py", line 318, in __init__
    self.locale = i18n.reset_locale(self)
  File "/usr/lib/ubiquity/ubiquity/i18n.py", line 35, in reset_locale
    di_locale = frontend.db.get('debian-installer/locale')
  File "/usr/lib/python3/dist-packages/debconf.py", line 83, in <lambda>
    lambda *args, **kw: self.command(command, *args, **kw))
  File "/usr/lib/python3/dist-packages/debconf.py", line 119, in command
    raise DebconfError(status, data)
debconf.DebconfError: (10, "debian-installer/locale doesn't exist")

Multiple re-images of the thumbdrive show the same error

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: ubiquity (not installed)
ProcVersionSignature: Ubuntu 4.18.0-10.11-generic 4.18.12
Uname: Linux 4.18.0-10-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CasperVersion: 1.394
CurrentDesktop: KDE
Date: Tue Jun 4 00:51:05 2019
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/kubuntu.seed boot=casper maybe-ubiquity quiet splash ---
LiveMediaBuild: Kubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.2)
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)