diff --git a/encodings.lisp b/encodings.lisp
index 04ddd93..bdd8e99 100644
--- a/encodings.lisp
+++ b/encodings.lisp
@@ -160,8 +160,10 @@
                  `((lambda (x)
                      (when (or (<= #xD800 x #xDBFF)
 			       (<= #xDC00 x #xDFFF))
-		       (xerror "surrogate encoded in UTF-8: #x~x." x))
-                     (cond ((%> x #xFFFF)
+		       (xerror "surrogate encoded in UTF-8: #x~X." x))
+                     (cond ((%> x #x10FFFF)
+                            (xerror "not a valid code point: #x~X" x))
+		           ((%> x #xFFFF)
                             (setf (aref out (%+ 0 wptr)) (%+ #xD7C0 (ash x -10))
                                   (aref out (%+ 1 wptr)) (%ior #xDC00 (%and x #x3FF)))
                             (setf wptr (%+ wptr 2)))