diff --git a/core/core-lib.el b/core/core-lib.el
index cb14d7326..5334b6d42 100644
--- a/core/core-lib.el
+++ b/core/core-lib.el
@@ -781,6 +781,9 @@ testing advice (when combined with `rotate-text').
 ;;; Backports
 
 (unless EMACS28+
+  ;; `format-spec' wasn't autoloaded until 28
+  (autoload #'format-spec "format-spec")
+
   (defun ensure-list (object)
     "Return OBJECT as a list.
 If OBJECT is already a list, return OBJECT itself.  If it's