In my custom installation shim-install failed during installation.  The only
information installer relayed was exit code 1.  After re-running shim-install
on the installed system I got the same result.

fmn:~ # shim-install --config-file=/boot/grub2/grub.cfg 
fmn:~ # echo $?
1

Running shim-install with Bash debugging enabled has shown the root cause.

fmn:~ # bash -x /usr/sbin/shim-install --config-file=/boot/grub2/grub.cfg 
...
+ bootdir=/boot
+ '[' -n '' ']'
+ test -n ''
+ test -d /boot/efi
+ test -d /boot/EFI
+ test -n ''
+ test -n ''
+ test -n ''
+ exit 1

I forgot to instruct the installer to mount EFI partition during installation. 
I would welcome a more verbose message from shim-install in this case for the
sake of reduced debugging time.