Failure to run grub-install on focal-arm64

The Focal installer fails to run install-grub on arm64. The relevant curtin log excerpt is:

curtin.util.ProcessExecutionError: Unexpected error while running command.
Command: ['sh', '-c', 'exec "$0" "$@" 2>&1', 'install-grub', '--uefi', '--update-nvram', '--os-family=debian', '/target', '/dev/vdb1']
Exit code: 1
Reason: -
Stdout: carryover command line params ''
        setting GRUB_CMDLINE_LINUX_DEFAULT to '' in etc/default/grub
        updated /target/etc/default/grub to set GRUB_CMDLINE_LINUX_DEFAULT=""
        basename: missing operand
        Try 'basename --help' for more information.

The failing basename command is in curtin's helpers/common at line 846. I'm attaching the full installer logs.

Note the presence of several errors like:

depmod: ERROR: ../libkmod/libkmod.c:515 lookup_builtin_file() could not open builtin file '/lib/modules/5.4.0-14-generic/modules.builtin.bin'

in the curtin log. I filed a different issue for these:

https://bugs.launchpad.net/curtin/+bug/1864992