Comment 11 for bug 1512347

@sschapiro: You can test this by using a 16.04 netboot image (http://cdimage.ubuntu.com/netboot/), and putting apt-setup/proposed=true on the installer's kernel command line or adding "d-i apt-setup/proposed boolean true" to your preseed file. (The result will be that the installed system has unverified updates on it too, so it would be best to only use this for testing this fix.)