diff --git a/lisp/nxml/nxml-mode.el b/lisp/nxml/nxml-mode.el
index 1eb728f23a5..5c906a9d510 100644
--- a/lisp/nxml/nxml-mode.el
+++ b/lisp/nxml/nxml-mode.el
@@ -544,6 +544,7 @@ Many aspects this mode can be customized using
   (setq-local syntax-ppss-table sgml-tag-syntax-table)
   (setq-local syntax-propertize-function #'nxml-syntax-propertize)
   (add-hook 'change-major-mode-hook #'nxml-cleanup nil t)
+  (add-hook 'after-change-functions #'nxml-maybe-rescan-prolog nil t)
 
   ;; Emacs 23 handles the encoding attribute on the xml declaration
   ;; transparently to nxml-mode, so there is no longer a need for the below
diff --git a/lisp/nxml/nxml-rap.el b/lisp/nxml/nxml-rap.el
index 21dbaded25a..3be413ae00b 100644
--- a/lisp/nxml/nxml-rap.el
+++ b/lisp/nxml/nxml-rap.el
@@ -108,6 +108,12 @@ Return nil if the character at POS is not inside."
     (setq nxml-prolog-regions (xmltok-forward-prolog))
     (setq nxml-prolog-end (point))))
 
+(defun nxml-maybe-rescan-prolog (start _end _length)
+  "Reparse the prolog if START lies within it.
+`nxml-mode' adds this function on `after-change-functions'."
+  (when (<= start nxml-prolog-end)
+    (save-excursion
+      (nxml-scan-prolog))))
 
 ;;; Random access parsing