I have a matrix-configuration job in which I am trying to copy artifacts from an upstream matrix-configuration job where some of the axes combinations of the upstream job are not run and therefore do not produce artifacts. In the job I am running the copy artifacts action I use upstream_job/arch=$arch,distro=$distro as the Project name so that each combination copies it's respective upstream artifacts.

However knowing that there upstream combinations that are not run and therefore don't have artifacts I have selected "Optional" for this copy artifacts action but it's still failing with:

Unable to find project for artifact copy: upstream/arch=x86_64,distro=el5
This may be due to incorrect project name or permission settings; see help for project name in job configuration.
Build step 'Copy artifacts from another project' marked build as failure

I can't simply exclude the same axes combinations in the job I am doing the artifact copying in as I do upstream since there are other actions that need to be done even when then are no artifacts from the upstream job.

Am I misunderstanding the use of Optional in the Copy Artifacts plugin?