diff --git a/lisp/textmodes/mhtml-ts-mode.el b/lisp/textmodes/mhtml-ts-mode.el
index bdb016d67dc..0d53176f166 100644
--- a/lisp/textmodes/mhtml-ts-mode.el
+++ b/lisp/textmodes/mhtml-ts-mode.el
@@ -315,7 +315,10 @@ NODE and PARENT are ignored."
      (defun ,(regexp-opt (list css--treesit-defun-type-regexp)))))
   "Settings for `treesit-thing-settings'.")
 
-(defvar mhtml-ts-mode--treesit-indent-rules
+;; We use a function instead of a variable, because doesn't exist when
+;; at compile time (unless we `eval-when-compile').
+(defun mhtml-ts-mode--treesit-indent-rules ()
+  "Return intent rules for `mhtml-ts-mode'."
   (treesit--indent-rules-optimize
    (append html-ts-mode--indent-rules
            ;; Extended rules for js and css, to
@@ -535,7 +538,8 @@ Powered by tree-sitter."
     ;; `mhtml-ts-mode-tag-relative-indent' and can be used to indent
     ;; JavaScript and CSS code relative to the HTML that contains them,
     ;; just like in mhtml-mode.
-    (setq-local treesit-simple-indent-rules mhtml-ts-mode--treesit-indent-rules)
+    (setq-local treesit-simple-indent-rules
+                (mhtml-ts-mode--treesit-indent-rules))
 
     ;; Navigation.