Can't install GRUB on RAID1

:~$ cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid1 sda[0] sdb[1]
      10274744 blocks super 1.2 [2/2] [UU]

unused devices: <none>
===============================================
~$ lsblk
NAME      MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sda         8:0    0   9.8G  0 disk
└─md0       9:0    0   9.8G  0 raid1
  ├─md0p1 259:0    0   350M  0 md    /boot
  ├─md0p2 259:1    0   1.5G  0 md    [SWAP]
  ├─md0p3 259:2    0     7G  0 md    /
  └─md0p4 259:3    0  1015M  0 md    /home
sdb         8:16   0   9.8G  0 disk
└─md0       9:0    0   9.8G  0 raid1
  ├─md0p1 259:0    0   350M  0 md    /boot
  ├─md0p2 259:1    0   1.5G  0 md    [SWAP]
  ├─md0p3 259:2    0     7G  0 md    /
  └─md0p4 259:3    0  1015M  0 md    /home
sr0        11:0    1 419.6M  0 rom