Reproduction steps for bug:

- emacs -Q
- (setq gnus-secondary-select-methods
    '((nnatom "github.com/vedang/pdf-tools/commits.atom")))
  (setq gnus-select-method '(nnnil ""))
- M-x gnus
- Open a message in the new group and press *
- Add the cache virtual server (info "(gnus) Creating a Virtual Server")
- ^ (server buffer) and: g on the cache
- RET to open (fails)

This is a possible fix that I've tested only on my (limited) setup, for
now: