18.04 - Custom install - ""No installable kernel was found in the defined APT sources"

I am trying to build a custom build of 18.04 with some extra packages installed, using https://help.ubuntu.com/community/InstallCDCustomization (substituting 'bionic' for 'xenial'),
but when installing on VM I get the message "No installable kernel was found in the defined APT sources".

Looking at the syslog, I see four warnings:

warning: Unable to find main/debian-installer/binary-amd64/Packages in /cdrom/dists/bionic/Release
warning: Unable to find restricted/debian-installer/binary-amd64/Packages in /cdrom/dists/bionic/Release
warning: Unable to find extras/debian-installer/binary-amd64/Packages in /cdrom/dists/bionic/Release
warning: Unable to find extras/debian-installer/binary-amd64/Packages.gz in /cdrom/dists/bionic/Release

And one message at the end:

base-installer: info: found kernels ''

It looks like the kernel packages are listed in dists/bionic/main/binary-amd64/Packages.gz, and the checksum from that file is correct in the dists/bionic/Release file.

What else might be missing?

EDIT:
Looking at the code of library.sh in the base-installer source code, that last message indicates that /tmp/available_kernels.txt is empty. And that indeed is the case when I quit out of the install w/Alt-F2.

EDIT 2: Looking further at library.sh, that file gets populated by the kernel_update_list function, which gets the list of kernel candidates from

chroot /target apt-cache search "^(kernel|$KERNEL_NAME)-image"

That returns nothing when I run it in the installer shell. I can see a number of other packages, including linux-headers-4.15.0-20-generic_4.15.0-20.21_amd64.deb, and the extra packages I'm trying to install. 'apt-cache search linux-headers' doesn't return anything, though.

EDIT 3: In pool/main/l/linux-signed is the linux-image-4.15.0-20-generic_4.15.0-20.21_amd64.deb.
Both standard 18.04 and my custom version have this. But I don't see where that package is listed under the dists/ directory.