Dear, Trying old Python 3.6, I just checked the old commit with “git log” and run the command: --8<---------------cut here---------------start------------->8--- guix time-machine --commit=ce35dc84a1 \ -- environment -C --ad-hoc python -- python --version --8<---------------cut here---------------end--------------->8--- which after burning some CPU fails! --8<---------------cut here---------------start------------->8--- […] 3507:20 4 (_) 2806:4 3 (save-module-excursion #<procedure 7f935c0495d0 at ice-…>) 3527:26 2 (_) In unknown file: 1 (primitive-load-path "gnu/packages/gnuzilla" #<procedur…>) In gnu/packages/gnuzilla.scm: 601:22 0 (_) gnu/packages/gnuzilla.scm:601:22: ERROR: 1. &message: "icecat-makeicecat.patch: patch not found" --8<---------------cut here---------------end--------------->8--- Well, it is expected because the commit is from March 2018 so before the big overhaul and all. But it is annoying to hit it, especially if there is some verification / authentication when “Updating channel”. Does it make to add an error if the commit is not reachable for “guix time-machine”? All the best, simon