foreign guix offload checklist?

[Jesse Gibbons <jgibbons2357@gmail.com>]

Hi guix,
I'm trying to offload a build onto a machine with a foreign distro. I'm 
getting the following error:

$ guix build --system=armhf-linux --no-substitutes hello
The following derivations will be built:
    /gnu/store/jidb0saw2s3zd1gvnziw064hbb1hxz7y-hello-2.10.drv
/gnu/store/hknqainda0ardh1ch1grrazislmmm1js-module-import-compiled.drv
    /gnu/store/w9lgwyp9a9v12x0bja3brhqj5ilb6l93-hello-2.10.tar.gz.drv
process 22691 acquired build slot '/var/guix/offload/192.168.1.28:22/0'
normalized load on machine '192.168.1.28' is 0.02
building 
/gnu/store/w9lgwyp9a9v12x0bja3brhqj5ilb6l93-hello-2.10.tar.gz.drv...
waiting for locks or build slots...
guix offload: error: failed to connect to `#<input-output: channel 
(open) 7f5b4a378460>': Protocol error
cannot build derivation 
`/gnu/store/jidb0saw2s3zd1gvnziw064hbb1hxz7y-hello-2.10.drv': 1 
dependencies couldn't be built
guix build: error: build of 
`/gnu/store/jidb0saw2s3zd1gvnziw064hbb1hxz7y-hello-2.10.drv' failed


Is there a checklist for what must be done to set up a machine with a 
foreign distro as a build machine ("build slave")? Here's what I have so 
far:

- [X] Make sure guile-ssh is installed on master. This should already be 
done if master runs guix distro.
- [X] Configure slave to accept master's ssh key '~/.ssh/id_rsa.pub`. If 
user wants to use configurations as close to default as possible, also 
configure it to accept '/root/.ssh/id_rsa.pub'.
- [X] install guix on master
- [X] Exchange guix public keys between master and slave. If master runs 
guix distro, the slave key should be added to the config.
- [X] Add slave to /etc/guix/machines.scm as described in the manual.

Am I missing any step?