How to add new support to `expand-region`?

Question

I'm trying to add new support to expand-region:

(defun er/lisp-mode ()
  (interactive)
  "Enhancement for lisp like #'() ,@()"
  (when (looking-at "\\(\\(`,?\\)\\|\\('?,?\\)\\|\\(,?@?\\)\\|\\(#?'?\\)\\)?(")
    (cond ((er/looking-back-on-line ",\\|@\\|?\\|#\\|'\\|`")
           (backward-char 1)))
    (set-mark (point))
    (forward-list)
    (exchange-point-and-mark)))

(defun er/add-lisp-mode-expansions ()
  (make-variable-buffer-local 'er/try-expand-list)
  (setq er/try-expand-list (append er/try-expand-list '(er/lisp-mode))))

(use-package expand-region
  :ensure t
  :bind
  ("C-SPC" . er/expand-region)
  :hook ((lisp-mode slime-mode) . er/add-lisp-mode-expansions))

When I open the first .lisp file, it looks fine. But after the first file, it doesn't work: expand-region will select all text of the entire buffer regardless of where I call er/expand-region.

When I look at er/try-expand-list in those buffers, it gives (er/lisp-mode er/lisp-mode) in lisp-mode buffers, and nil in buffers of other modes.

As far as I can see, it is er/try-expand-list that controls the behaviour of er/expand-region, so the problem is the wrong value of er/try-expand-region, which should be composed from the original value provided by expand-region with/without er/lisp-mode added, depending on the mode of the buffer.

How should I fix it?