diff --git a/module/mes/M1.mes b/module/mes/M1.mes
index 908312a8..3c68d69f 100644
--- a/module/mes/M1.mes
+++ b/module/mes/M1.mes
@@ -150,7 +150,8 @@
         (cond ((and (< len string-max)
                     (char? (car data))
                     (eq? (last data) #\nul)
-                    (not (find (cut memq <> '(#\nul #\backspace #\return #\" #\')) (list-head data (1- (length data))))))
+                    (not (find (cut memq <> '(#\nul #\backspace #\return #\" #\')) (list-head data (1- (length data)))))
+                    (not (any (lambda (ch) (>= (char->integer ch) #x80)) data)))
                (format #t "\"~a\"" (list->string (list-head data (1- (length data))))))
               (else (format #t "~a" (string-join (map text->M1 data) " "))))
         (newline)))