Kubuntu installer crashes during partitioning

STEPS TO REPRODUCE:
1. Prior to installer invocation, have an ext4 logical volume configured on an LVM volume group.
2. Choose manual partitioning
3. Click on the ext4 volume
4. Click change
5. Specify a mount path, but don't check the box to format.
6. Click OK
7. Click the box in the "Format?" column for the same logical volume.
8. Installer crashes with the stack trace below.

Traceback (most recent call last):
  File "/usr/lib/ubiquity/ubiquity/frontend/kde_components/PartitionModel.py", line 66, in setData
    item.partman_column_format_toggled(value.toBool())
AttributeError: 'int' object has no attribute 'toBool'

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.3
ProcVersionSignature: Ubuntu 4.15.0-10.11-generic 4.15.3
Uname: Linux 4.15.0-10-generic x86_64
ApportVersion: 2.20.8-0ubuntu10
Architecture: amd64
CasperVersion: 1.389
Date: Wed Mar 21 20:56:13 2018
InstallCmdLine: file=/cdrom/preseed/kubuntu.seed boot=casper maybe-ubiquity initrd=/casper/initrd.lz quiet splash --- maybe-ubiquity
LiveMediaBuild: Kubuntu 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180306.1)
ProcEnviron:
 LANGUAGE=
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)