I have installed LMDE (with grub in MBR) after I installed Windows 8.1. I then installed the grub-efi package and added the custom Windows options:

#!/bin/sh
exec tail -n +3 $0

menuentry "Windows" {
    search --fs-uuid --no-floppy --set=root A89A-7F4C
    chainloader (${root})/EFI/Boot/bkpbootx64.efi
}
menuentry "Windows (backup bootloader)" {
    search --fs-uuid --no-floppy --set=root A89A-7F4C
    chainloader (${root})/EFI/Microsoft/Boot/bkpbootmgfw.efi
}

These are basically a leftover from my older Ubuntu setup. However, grub is refusing to load them, complaining about the invalid signature. What do I do now?