Stack Exchange network consists of 183 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
This GitHub issue thread suggests that this was a known issue for a while, but the bug was fixed. Can you pull the latest version of undo-tree.el from MELPA, or failing that, straight from GH, and see if it goes away?
undo-tree.elfrom MELPA, or failing that, straight from GH, and see if it goes away?