Customizing Trisquel .iso -- problem w/ chroot

I'm trying to customize a Trisquel .iso (trisquel-mini_5.0_amd64.iso), as per the "Advanced" section of the manual. But when I enter this command:

chroot jail


I get this error:

chroot: failed to run command `/bin/bash': Exec format error


What should I do to change the root directory? There's nothing about this in the man page, or any other documentation I could find.

Thanks,
Chris Adams.