UCK failed with Ubuntu 20.04 ISO when run on 20.04 and 18.04

Here is the build log file from my 20.04 machine. Note that the error was the same on 16.04.

Build (/usr/bin/uck-gui --wait-before-exit) started at 2020-06-17 12:57:20
>> Ubuntu Customization Kit 2.4.7 on Ubuntu 20.04, 5.4.0-37-generic x86_64
Starting CD remastering on Wed 17 Jun 2020 12:57:23 PM EDT
Customization dir=/home/alan/tmp/customization-scripts
Mounting ISO image...
mount: /home/alan/tmp/remaster-iso-mount: WARNING: device write-protected, mounted read-only.
Unpacking ISO image...
Unmounting ISO image...
Mounting SquashFS image...
Unpacking SquashFS image...
Unmounting SquashFS image...
Removing win32 files...
Creating apt cache...
Creating root home...
Mounting /proc
Mounting /sys
Mounting /dev/pts
Mounting /tmp
Mounting /home/alan/tmp/remaster-root-home
Mounting /home/alan/tmp/remaster-apt-cache
Mounting /run
Mounting /home/alan/tmp/customization-scripts
Copying fstab/mtab...
cp: '/etc/mtab' and '/home/alan/tmp/remaster-root/etc/mtab' are the same file
Unmounting /home/alan/tmp/remaster-root/var/cache/apt...
Unmounting /home/alan/tmp/remaster-root/tmp/customization-scripts...
Unmounting /home/alan/tmp/remaster-root/tmp...
Unmounting /home/alan/tmp/remaster-root/sys...
Unmounting /home/alan/tmp/remaster-root/run...
Unmounting /home/alan/tmp/remaster-root/root...
Unmounting /home/alan/tmp/remaster-root/proc...
Unmounting /home/alan/tmp/remaster-root/dev/pts...
Failed to copy mtab, error=1