Failure to create whole-disk image using the RHEL 7.6 guest image

Issue

2018-11-13 18:11:27.801 | INFO diskimage_builder.block_device.utils [-] Calling [sudo fstrim --verbose /tmp/dib_build.qLQK2n9c/mnt/tmp]
2018-11-13 18:11:27.845 | INFO diskimage_builder.block_device.utils [-] Calling [sudo umount /tmp/dib_build.qLQK2n9c/mnt/tmp]
2018-11-13 18:11:33.206 | Traceback (most recent call last):
2018-11-13 18:11:33.208 |   File "/usr/bin/dib-block-device", line 10, in <module>
2018-11-13 18:11:33.208 |     sys.exit(main())
2018-11-13 18:11:33.208 |   File "/usr/lib/python2.7/site-packages/diskimage_builder/block_device/cmd.py", line 120, in main
2018-11-13 18:11:33.208 |     return bdc.main()
2018-11-13 18:11:33.208 |   File "/usr/lib/python2.7/site-packages/diskimage_builder/block_device/cmd.py", line 115, in main
2018-11-13 18:11:33.208 |     self.args.func()
2018-11-13 18:11:33.208 |   File "/usr/lib/python2.7/site-packages/diskimage_builder/block_device/cmd.py", line 39, in cmd_umount
2018-11-13 18:11:33.208 |     self.bd.cmd_umount()
2018-11-13 18:11:33.208 |   File "/usr/lib/python2.7/site-packages/diskimage_builder/block_device/blockdevice.py", line 441, in cmd_umount
2018-11-13 18:11:33.208 |     node.umount()
2018-11-13 18:11:33.208 |   File "/usr/lib/python2.7/site-packages/diskimage_builder/block_device/level3/mount.py", line 112, in umount
2018-11-13 18:11:33.208 |     exec_sudo(["umount", self.state['mount'][self.mount_point]['path']])
2018-11-13 18:11:33.208 |   File "/usr/lib/python2.7/site-packages/diskimage_builder/block_device/utils.py", line 125, in exec_sudo
2018-11-13 18:11:33.208 |     ' '.join(sudo_cmd))
2018-11-13 18:11:33.208 | subprocess.CalledProcessError: Command 'sudo umount /tmp/dib_build.qLQK2n9c/mnt/tmp' returned non-zero exit status 32