kernel.txz checksum mismatch when installing via bsdinstall's installerconfig

[AppAraat]

Hi,

I'm trying to do an unattended install of a FreeBSD system.
The plan is to be able to physically connect a USB stick which has an image on it (which has a custom installerconfig) to a machine, boot off of it and let bsdinstall do its thing.

However, I think I'm a bit confused regarding exact syntax of my installerconfig file. When I boot into a VM to test my custom image, at some point there's an error:

The checksum for kernel.txz does not match. It may have become corrupted, and should be redownloaded.

And then I'm greeted with a root prompt.

I'm trying to keep things simple for now and as such I'm ok with most of the defaults. Here's the installerconfig I'm trying to go with:

export DISTRIBUTIONS="kernel.txz base.txz doc.txz"
export BSDINSTALL_DISTSITE=ftp://ftp.freebsd.org/pub/FreeBSD/releases/amd64/amd64/12.0-RELEASE
export INTERFACES=re0 # VM test
# Comment out the line above and uncomment line below if you're going to do this on the real deal.
#export INTERFACES=em1

export nonInteractive="YES"
#export ZFSBOOT_DISKS="da0 da1" # this is for realsies
export ZFSBOOT_DISKS="ada0 ada1" # VM test
export ZFSBOOT_VDEV_TYPE="mirror" 
export ZFSBOOT_SWAP_MIRROR="1" 
export ZFSBOOT_SWAP_ENCRYPTION="YES"

#!/bin/sh
echo "sshd_enable=YES" >> /etc/rc.conf
echo "ifconfig_AUTO=DHCP" >> /etc/rc.conf

The image I'm customizing is FreeBSD-12.0-RELEASE-amd64-bootonly.iso (with a custom installerconfig residing in /etc/installerconfig being the only "non-standard" file)

I'm not sure what is going awry so if anyone can provide some helpful feedback, that would be much appreciated.

Thanks,
Appa