Can't do-release-upgrade -d from 16.04 to 18.04

Question

Tried do-release-upgrade -d on a 16.04 machine today but got

Upgrades to the development release are only 
available from the latest supported release.

Surely LTS to LTS is allowed?

Result of cat /etc/update-manager/release-upgrades is Prompt=lts

Answer 1

Remove any files in ~/.cache/update-manager-core/ and try the upgrade process again.