diff --git a/lisp/font-lock.el b/lisp/font-lock.el
index ef0e2dc0458..4eb7bb8c2d4 100644
--- a/lisp/font-lock.el
+++ b/lisp/font-lock.el
@@ -1106,8 +1106,10 @@ This functions is a convenience functions when developing font
 locking for a mode, and is not meant to be called from lisp functions."
   (interactive)
   (declare (interactive-only t))
-  (setq font-lock-major-mode nil
-        syntax-propertize--done -1)
+  ;; Make font-lock recalculate all the mode-specific data.
+  (setq font-lock-major-mode nil)
+  ;; Make the syntax machinery discard all information.
+  (syntax-ppss-flush-cache -1)
   (font-lock-set-defaults)
   (save-excursion
     (font-lock-fontify-region (point-min) (point-max))))