linux-imageの削除と更新中にエラーが発生しました。

Elementary OSを更新しようとするたびに、次のエラーが発生します。

Removing linux-image-extra-3.13.0-32-generic (3.13.0-32.57) ...
Examining /etc/kernel/postrm.d .
run-parts: executing /etc/kernel/postrm.d/initramfs-tools 3.13.0-32-generic /boot/vmlinuz-3.13.0-32-generic
update-initramfs: Deleting /boot/initrd.img-3.13.0-32-generic
run-parts: executing /etc/kernel/postrm.d/zz-update-grub 3.13.0-32-generic /boot/vmlinuz-3.13.0-32-generic
/usr/sbin/grub-mkconfig: 11: /etc/default/grub: GRUB: not found
run-parts: /etc/kernel/postrm.d/zz-update-grub exited with return code 127
Failed to process /etc/kernel/postrm.d at /var/lib/dpkg/info/linux-image-extra-3.13.0-32-generic.postrm line 328.
dpkg: error processing package linux-image-extra-3.13.0-32-generic (--remove):
 subprocess installed post-removal script returned error exit status 1
Removing linux-image-3.13.0-32-generic (3.13.0-32.57) ...
Examining /etc/kernel/postrm.d .
run-parts: executing /etc/kernel/postrm.d/initramfs-tools 3.13.0-32-generic /boot/vmlinuz-3.13.0-32-generic
update-initramfs: Deleting /boot/initrd.img-3.13.0-32-generic
run-parts: executing /etc/kernel/postrm.d/zz-update-grub 3.13.0-32-generic /boot/vmlinuz-3.13.0-32-generic
/usr/sbin/grub-mkconfig: 11: /etc/default/grub: GRUB: not found
run-parts: /etc/kernel/postrm.d/zz-update-grub exited with return code 127
Failed to process /etc/kernel/postrm.d at /var/lib/dpkg/info/linux-image-3.13.0-32-generic.postrm line 328.
dpkg: error processing package linux-image-3.13.0-32-generic (--remove):
 subprocess installed post-removal script returned error exit status 1
Errors were encountered while processing:
 linux-image-extra-3.13.0-32-generic
 linux-image-3.13.0-32-generic
E: Sub-process /usr/bin/dpkg returned an error code (1)

パッケージを手動で削除しようとしましたが、エラーが返されました。