The error suggests that rake tasks cannot return so I think the merged fix referenced earlier is incorrect. I don’t have an environment set up to do any testing or create a pull request but I think using the following instead would fix this. (cc @martin)
Note that rebuilding right now (at time of writing) will still fail if you’re on tests-passed as the tests haven’t completed yet. If you look at the commit page, you’ll see an orange dot to the left of the title indicating that the tests are still in progress:
When that changes to a green tick or (more likely) a red cross, the tests are complete. Then you can rebuild. (An experimental test will probably fail, hence the red cross, but that doesn’t affect tests-passed.)
Edit: Looking at the tests-passed branch as @CvX suggests is more useful. Looks like it’s good to go now.