Failed to install GRUB/sparc64 on an md softraid device

[Koakuma]

I am trying to set up GRUB on a softraid md device on Debian. I've created an
md device on top of /dev/sdb1, formatted it, and mounted the device on /boot,
but when I'm about to install GRUB into the partition, I got an error like
the one shown below.
Am I doing anything wrong here? What's the proper way to install GRUB/sparc64
on an md device?

This is the error I got when trying to install GRUB:

$ sudo grub-install --force --skip-fs-probe /dev/sdb1
Installing for sparc64-ieee1275 platform.
grub-install: warning: Couldn't find physical volume `(null)'. Some modules may 
be missing from core image..
grub-install: warning: Couldn't find physical volume `(null)'. Some modules may 
be missing from core image..
grub-install: warning: Discarding improperly nested partition 
(hostdisk//dev/sdb,sun1,sun2).
grub-install: warning: Discarding improperly nested partition 
(hostdisk//dev/sdb,sun1,sun4).
grub-install: warning: Attempting to install GRUB to a disk with multiple 
partition labels.  This is not supported yet..
grub-install: error: embedding is not possible, but this is required for RAID 
and LVM install.

Installing to /dev/md0 also gives the same error message.

This is the partition table of /dev/sdb:

$ sudo fdisk -l /dev/sdb
Disk /dev/sdb: 136.75 GiB, 146810536448 bytes, 286739329 sectors
Disk model: H103014SCSUN146G
Geometry: 255 heads, 63 sectors/track, 17848 cylinders
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: sun

Device       Start       End   Sectors   Size Id Type         Flags
/dev/sdb1        0    979964    979965 478.5M 83 Linux native
/dev/sdb2   979965   8787554   7807590   3.7G 82 Linux swap
/dev/sdb3        0 286728119 286728120 136.7G  5 Whole disk
/dev/sdb4  8787555 286728119 277940565 132.5G 83 Linux native