Path `/boot/grub' is not readable by GRUB on boot. Installation is impossible. Aborting

Whenever I try updating grub, I get this error:

# grub-install /dev/sda
Path `/boot/grub' is not readable by GRUB on boot. Installation is impossible. Aborting.

and

# grub-mkconfig -o /boot/grub/grub.cfg 
/usr/sbin/grub-probe: error: cannot find a device for / (is /dev mounted?).

Also, I'm using the btrfs.