Package fetch error: Could not fetch linux-base 4.5ubuntu3.7 source package: Source '<unkown>' is not trusted

I’m trying to create a kernel snap for an arm64 device using the focal HWE kernel, but it’s failing with the following error message:

Get:1 http://ports.ubuntu.com focal InRelease [265 kB]
Hit:2 http://archive.ubuntu.com/ubuntu focal InRelease
Hit:3 http://archive.ubuntu.com/ubuntu focal-updates InRelease
Hit:4 http://archive.ubuntu.com/ubuntu focal-backports InRelease
Hit:5 http://ports.ubuntu.com focal-updates InRelease
Hit:6 http://security.ubuntu.com/ubuntu focal-security InRelease
Get:7 http://ports.ubuntu.com focal/main arm64 Packages [940 kB]
Get:8 http://ports.ubuntu.com focal/main Translation-en [506 kB]
Get:9 http://ports.ubuntu.com focal/main amd64 c-n-f Metadata [29.5 kB]
Get:10 http://ports.ubuntu.com focal/universe arm64 Packages [8458 kB]
Get:11 http://ports.ubuntu.com focal/universe Translation-en [5124 kB]
Get:12 http://ports.ubuntu.com focal/universe amd64 c-n-f Metadata [265 kB]
Get:13 http://ports.ubuntu.com focal-updates/main arm64 Packages [1063 kB]
Get:14 http://ports.ubuntu.com focal-updates/main Translation-en [296 kB]
Get:15 http://ports.ubuntu.com focal-updates/main amd64 c-n-f Metadata [14.7 kB]
Fetched 17.0 MB in 2s (8871 kB/s)
Reading package lists... Done
Get:1 iw_5.4-1_arm64.deb [90.5 kB]
Fetched 90.5 kB in 0s (0 B/s)
Get:1 linux-modules-extra-5.13.0-27-generic_5.13.0-27.29~20.04.1_arm64.deb [53.4 MB]
Fetched 53.4 MB in 0s (0 B/s)
Package fetch error: Could not fetch linux-base 4.5ubuntu3.7 source package: Source '<unkown>' is not trusted

It fetches the first couple of packages and then fails when fetching linux-base, for some reason.

I’m building on amd64 and calling snapcraft with the following command line:

snapcraft --destructive-mode --target-arch=arm64 --enable-experimental-target-arch 

The relevant parts of my snapcraft.yaml:

architectures:
  - build-on:
      - amd64
      - arm64
    run-on: arm64

package-repositories:
  - type: apt
    architectures:
      - arm64
    components:
      - main
    suites:
      - focal
      - focal-updates
    key-id: F6ECB3762474EDA9D21B7022871920D1991BC93C
    url: http://ports.ubuntu.com/

parts:
  kernel:
    plugin: nil
    stage-packages:
      - linux-image-generic-hwe-20.04
    override-build: |
      echo todo

Can anybody shed some light on to why this one package is failing? Or should I be downloading the latest kernel image another way? Unfortunately I cannot use the pc-kernel snap as it’s too old.

Also: “unkown” is spelled wrong, but I can’t find where that comes from.

Edit: Running this natively on arm64 works fine - it’s just a problem when cross-compiling.