[warn] assign_to_cpuworker failed. Ignoring.

On my guard relay running (a few commits before) 0.3.1.1-alpha, I have these log lines:

May 22 06:04:15.536 [warn] assign_to_cpuworker failed. Ignoring.
May 22 12:10:11.926 [warn] assign_to_cpuworker failed. Ignoring.

I don't have any more verbose logging for this one, but I'll try to get it.

I wonder what is causing it? If it's a bug, we should fix it. If it's really something to ignore, we should downgrade it.