grub2 - `install-grub` claims that I have multiple partiton labels and that embedding is impossible

I am trying to get GRUB installed on my non-booting desktop computer, but I am encountering several errors. Other sources discussing these errors have ascribed them to either a lack of free disk space before the first partition (I have the requisite free space), or to a problem with /boot/grub/grub.cfg (problem persists even after regenerating that file correctly).

I booted with a live USB and attempted to reinstall GRUB using the following commands, but I encounter an error that prevents it from working:

$ sudo mount /dev/sda1 /mnt
$ sudo grub-install --root-directory=/mnt /dev/sda
Installing for i386-pc platform.

grub-install: warning: Attempting to install GRUB to a disk with multiple partition labels.  This is not supported yet..
grub-install: warning: Embedding is not possible.  GRUB can only be installed in this setup by using blocklists.  However, blocklists are UNRELIABLE and their use is discouraged..
grub-install: error: will not proceed with blocklists.

However, /dev/sda appears to be formatted correctly for installing grub:

$ sudo fdisk -l /dev/sda


Disk /dev/sda: 111.8 GiB, 120034123776 bytes, 234441648 sectors
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: dos
Disk identifier: 0x8d91017b

Device     Boot Start       End   Sectors   Size Id Type
/dev/sda1  *     2048 234440703 234438656 111.8G 83 Linux


$ blkid /dev/sda1
/dev/sda1: UUID="84e9ff65-c4ba-42eb-8a6d-ebc703fae1f7" TYPE="ext4" PARTUUID="8d91017b-01"

It has the standard 1 MiB free space at the beginning, and /dev/sda1 is formatted correctly. I have tried using grub-mkconfig to rebuild the config file:

$ for f in proc sys dev dev/pts ; do sudo mount --bind /$f /mnt/$f ; done
$ sudo chroot /mnt
# grub-mkconfig -o /boot/grub/grub.cfg

Generating grub configuration file ...
Found linux image: /boot/vmlinuz-3.19.0-26-generic
Found initrd image: /boot/initrd.img-3.19.0-26-generic
Found linux image: /boot/vmlinuz-3.19.0-23-generic
Found initrd image: /boot/initrd.img-3.19.0-23-generic
Adding boot menu entry for EFI firmware configuration
done

However, I still get the same error message when running grub-install, whether in or out of the chroot environment.

Booting the live USB in legacy mode, I get the same error; the only difference is the output of running grub-mkconfig in the chroot environment:

# grub-mkconfig -o /boot/grub/grub.cfg
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-3.19.0-26-generic
Found initrd image: /boot/initrd.img-3.19.0-26-generic
Found linux image: /boot/vmlinuz-3.19.0-23-generic
Found initrd image: /boot/initrd.img-3.19.0-23-generic

Found memtest86+ image: /boot/memtest86+.elf
Found memtest86+ image: /boot/memtest86+.bin
Found Windows 7 (loader) on /dev/sdc1
done

How can I get grub installed correctly?