As noted in this issue comment, recent changes to the internals of undercover.el appear to have invalidated some assumptions of Eldev in a way that makes undercover unusable from Eldev. Attempting to do so causes an error.
(Note that the linked issue is about an unrelated issue in undercover. While attempting to test a fix for that issue, I ran into this issue.)
I fixed this half an hour ago in commit fb72721, as I saw the ping. Still goes into internals, but undercover currently gives no other option. Hopefully this will change at some point.
As noted in this issue comment, recent changes to the internals of undercover.el appear to have invalidated some assumptions of Eldev in a way that makes undercover unusable from Eldev. Attempting to do so causes an error.
(Note that the linked issue is about an unrelated issue in undercover. While attempting to test a fix for that issue, I ran into this issue.)